feat: oauth2 - add authorization server metadata endpoint and PKCE support (#18548)

## Summary

  This PR implements critical MCP OAuth2 compliance features for Coder's authorization server, adding PKCE support, resource parameter handling, and OAuth2 server metadata discovery. This brings Coder's OAuth2 implementation significantly closer to production readiness for MCP (Model Context Protocol)
  integrations.

  ## What's Added

  ### OAuth2 Authorization Server Metadata (RFC 8414)
  - Add `/.well-known/oauth-authorization-server` endpoint for automatic client discovery
  - Returns standardized metadata including supported grant types, response types, and PKCE methods
  - Essential for MCP client compatibility and OAuth2 standards compliance

  ### PKCE Support (RFC 7636)
  - Implement Proof Key for Code Exchange with S256 challenge method
  - Add `code_challenge` and `code_challenge_method` parameters to authorization flow
  - Add `code_verifier` validation in token exchange
  - Provides enhanced security for public clients (mobile apps, CLIs)

  ### Resource Parameter Support (RFC 8707)
  - Add `resource` parameter to authorization and token endpoints
  - Store resource URI and bind tokens to specific audiences
  - Critical for MCP's resource-bound token model

  ### Enhanced OAuth2 Error Handling
  - Add OAuth2-compliant error responses with proper error codes
  - Use standard error format: `{"error": "code", "error_description": "details"}`
  - Improve error consistency across OAuth2 endpoints

  ### Authorization UI Improvements
  - Fix authorization flow to use POST-based consent instead of GET redirects
  - Remove dependency on referer headers for security decisions
  - Improve CSRF protection with proper state parameter validation

  ## Why This Matters

  **For MCP Integration:** MCP requires OAuth2 authorization servers to support PKCE, resource parameters, and metadata discovery. Without these features, MCP clients cannot securely authenticate with Coder.

  **For Security:** PKCE prevents authorization code interception attacks, especially critical for public clients. Resource binding ensures tokens are only valid for intended services.

  **For Standards Compliance:** These are widely adopted OAuth2 extensions that improve interoperability with modern OAuth2 clients.

  ## Database Changes

  - **Migration 000343:** Adds `code_challenge`, `code_challenge_method`, `resource_uri` to `oauth2_provider_app_codes`
  - **Migration 000343:** Adds `audience` field to `oauth2_provider_app_tokens` for resource binding
  - **Audit Updates:** New OAuth2 fields properly tracked in audit system
  - **Backward Compatibility:** All changes maintain compatibility with existing OAuth2 flows

  ## Test Coverage

  - Comprehensive PKCE test suite in `coderd/identityprovider/pkce_test.go`
  - OAuth2 metadata endpoint tests in `coderd/oauth2_metadata_test.go`
  - Integration tests covering PKCE + resource parameter combinations
  - Negative tests for invalid PKCE verifiers and malformed requests

  ## Testing Instructions

  ```bash
  # Run the comprehensive OAuth2 test suite
  ./scripts/oauth2/test-mcp-oauth2.sh

  Manual Testing with Interactive Server

  # Start Coder in development mode
  ./scripts/develop.sh

  # In another terminal, set up test app and run interactive flow
  eval $(./scripts/oauth2/setup-test-app.sh)
  ./scripts/oauth2/test-manual-flow.sh
  # Opens browser with OAuth2 flow, handles callback automatically

  # Clean up when done
  ./scripts/oauth2/cleanup-test-app.sh

  Individual Component Testing

  # Test metadata endpoint
  curl -s http://localhost:3000/.well-known/oauth-authorization-server | jq .

  # Test PKCE generation
  ./scripts/oauth2/generate-pkce.sh

  # Run specific test suites
  go test -v ./coderd/identityprovider -run TestVerifyPKCE
  go test -v ./coderd -run TestOAuth2AuthorizationServerMetadata
```

  ### Breaking Changes

  None. All changes maintain backward compatibility with existing OAuth2 flows.

---

Change-Id: Ifbd0d9a543d545f9f56ecaa77ff2238542ff954a
Signed-off-by: Thomas Kosiewski <tk@coder.com>

coderd/httpmw/oauth2.go

@@ -207,6 +207,67 @@ func OAuth2ProviderApp(r *http.Request) database.OAuth2ProviderApp {
 // middleware requires the API key middleware higher in the call stack for
 // authentication.
 func ExtractOAuth2ProviderApp(db database.Store) func(http.Handler) http.Handler {
+	return extractOAuth2ProviderAppBase(db, &codersdkErrorWriter{})
+}
+
+// ExtractOAuth2ProviderAppWithOAuth2Errors is the same as ExtractOAuth2ProviderApp but
+// returns OAuth2-compliant errors instead of generic API errors. This should be used
+// for OAuth2 endpoints like /oauth2/tokens.
+func ExtractOAuth2ProviderAppWithOAuth2Errors(db database.Store) func(http.Handler) http.Handler {
+	return extractOAuth2ProviderAppBase(db, &oauth2ErrorWriter{})
+}
+
+// errorWriter interface abstracts different error response formats.
+// This uses the Strategy pattern to avoid a control flag (useOAuth2Errors bool)
+// which was flagged by the linter as an anti-pattern. Instead of duplicating
+// the entire function logic or using a boolean parameter, we inject the error
+// handling behavior through this interface.
+type errorWriter interface {
+	writeMissingClientID(ctx context.Context, rw http.ResponseWriter)
+	writeInvalidClientID(ctx context.Context, rw http.ResponseWriter, err error)
+	writeInvalidClient(ctx context.Context, rw http.ResponseWriter)
+}
+
+// codersdkErrorWriter writes standard codersdk errors for general API endpoints
+type codersdkErrorWriter struct{}
+
+func (*codersdkErrorWriter) writeMissingClientID(ctx context.Context, rw http.ResponseWriter) {
+	httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+		Message: "Missing OAuth2 client ID.",
+	})
+}
+
+func (*codersdkErrorWriter) writeInvalidClientID(ctx context.Context, rw http.ResponseWriter, err error) {
+	httpapi.Write(ctx, rw, http.StatusUnauthorized, codersdk.Response{
+		Message: "Invalid OAuth2 client ID.",
+		Detail:  err.Error(),
+	})
+}
+
+func (*codersdkErrorWriter) writeInvalidClient(ctx context.Context, rw http.ResponseWriter) {
+	httpapi.Write(ctx, rw, http.StatusUnauthorized, codersdk.Response{
+		Message: "Invalid OAuth2 client.",
+	})
+}
+
+// oauth2ErrorWriter writes OAuth2-compliant errors for OAuth2 endpoints
+type oauth2ErrorWriter struct{}
+
+func (*oauth2ErrorWriter) writeMissingClientID(ctx context.Context, rw http.ResponseWriter) {
+	httpapi.WriteOAuth2Error(ctx, rw, http.StatusBadRequest, "invalid_request", "Missing client_id parameter")
+}
+
+func (*oauth2ErrorWriter) writeInvalidClientID(ctx context.Context, rw http.ResponseWriter, _ error) {
+	httpapi.WriteOAuth2Error(ctx, rw, http.StatusUnauthorized, "invalid_client", "The client credentials are invalid")
+}
+
+func (*oauth2ErrorWriter) writeInvalidClient(ctx context.Context, rw http.ResponseWriter) {
+	httpapi.WriteOAuth2Error(ctx, rw, http.StatusUnauthorized, "invalid_client", "The client credentials are invalid")
+}
+
+// extractOAuth2ProviderAppBase is the internal implementation that uses the strategy pattern
+// instead of a control flag to handle different error formats.
+func extractOAuth2ProviderAppBase(db database.Store, errWriter errorWriter) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
 			ctx := r.Context()
@@ -233,26 +294,21 @@ func ExtractOAuth2ProviderApp(db database.Store) func(http.Handler) http.Handler
 					}
 				}
 				if paramAppID == "" {
-					httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
-						Message: "Missing OAuth2 client ID.",
-					})
+					errWriter.writeMissingClientID(ctx, rw)
 					return
 				}
 
 				var err error
 				appID, err = uuid.Parse(paramAppID)
 				if err != nil {
-					httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
-						Message: "Invalid OAuth2 client ID.",
-						Detail:  err.Error(),
-					})
+					errWriter.writeInvalidClientID(ctx, rw, err)
 					return
 				}
 			}
 
 			app, err := db.GetOAuth2ProviderAppByID(ctx, appID)
 			if httpapi.Is404Error(err) {
-				httpapi.ResourceNotFound(rw)
+				errWriter.writeInvalidClient(ctx, rw)
 				return
 			}
 			if err != nil {