feat(cli): add coder exp mcp command (#17066)

Adds a `coder exp mcp` command which will start a local MCP server
listening on stdio with the following capabilities:
* Show logged in user (`coder whoami`)
* List workspaces (`coder list`)
* List templates (`coder templates list`)
* Start a workspace (`coder start`)
* Stop a workspace (`coder stop`)
* Fetch a single workspace (no direct CLI analogue)
* Execute a command inside a workspace (`coder exp rpty`)
* Report the status of a task (currently a no-op, pending task support)

This can be tested as follows:

```
# Start a local Coder server.
./scripts/develop.sh
# Start a workspace. Currently, creating workspaces is not supported.
./scripts/coder-dev.sh create -t docker --yes
# Add the MCP to your Claude config.
claude mcp add coder ./scripts/coder-dev.sh exp mcp
# Tell Claude to do something Coder-related. You may need to nudge it to use the tools.
claude 'start a docker workspace and tell me what version of python is installed'
```

cli/exp_mcp_test.go

@@ -0,0 +1,142 @@
+package cli_test
+
+import (
+	"context"
+	"encoding/json"
+	"runtime"
+	"slices"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/cli/clitest"
+	"github.com/coder/coder/v2/coderd/coderdtest"
+	"github.com/coder/coder/v2/pty/ptytest"
+	"github.com/coder/coder/v2/testutil"
+)
+
+func TestExpMcp(t *testing.T) {
+	t.Parallel()
+
+	// Reading to / writing from the PTY is flaky on non-linux systems.
+	if runtime.GOOS != "linux" {
+		t.Skip("skipping on non-linux")
+	}
+
+	t.Run("AllowedTools", func(t *testing.T) {
+		t.Parallel()
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+		cancelCtx, cancel := context.WithCancel(ctx)
+		t.Cleanup(cancel)
+
+		// Given: a running coder deployment
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+
+		// Given: we run the exp mcp command with allowed tools set
+		inv, root := clitest.New(t, "exp", "mcp", "server", "--allowed-tools=coder_whoami,coder_list_templates")
+		inv = inv.WithContext(cancelCtx)
+
+		pty := ptytest.New(t)
+		inv.Stdin = pty.Input()
+		inv.Stdout = pty.Output()
+		clitest.SetupConfig(t, client, root)
+
+		cmdDone := make(chan struct{})
+		go func() {
+			defer close(cmdDone)
+			err := inv.Run()
+			assert.NoError(t, err)
+		}()
+
+		// When: we send a tools/list request
+		toolsPayload := `{"jsonrpc":"2.0","id":2,"method":"tools/list"}`
+		pty.WriteLine(toolsPayload)
+		_ = pty.ReadLine(ctx) // ignore echoed output
+		output := pty.ReadLine(ctx)
+
+		cancel()
+		<-cmdDone
+
+		// Then: we should only see the allowed tools in the response
+		var toolsResponse struct {
+			Result struct {
+				Tools []struct {
+					Name string `json:"name"`
+				} `json:"tools"`
+			} `json:"result"`
+		}
+		err := json.Unmarshal([]byte(output), &toolsResponse)
+		require.NoError(t, err)
+		require.Len(t, toolsResponse.Result.Tools, 2, "should have exactly 2 tools")
+		foundTools := make([]string, 0, 2)
+		for _, tool := range toolsResponse.Result.Tools {
+			foundTools = append(foundTools, tool.Name)
+		}
+		slices.Sort(foundTools)
+		require.Equal(t, []string{"coder_list_templates", "coder_whoami"}, foundTools)
+	})
+
+	t.Run("OK", func(t *testing.T) {
+		t.Parallel()
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+		cancelCtx, cancel := context.WithCancel(ctx)
+		t.Cleanup(cancel)
+
+		client := coderdtest.New(t, nil)
+		_ = coderdtest.CreateFirstUser(t, client)
+		inv, root := clitest.New(t, "exp", "mcp", "server")
+		inv = inv.WithContext(cancelCtx)
+
+		pty := ptytest.New(t)
+		inv.Stdin = pty.Input()
+		inv.Stdout = pty.Output()
+		clitest.SetupConfig(t, client, root)
+
+		cmdDone := make(chan struct{})
+		go func() {
+			defer close(cmdDone)
+			err := inv.Run()
+			assert.NoError(t, err)
+		}()
+
+		payload := `{"jsonrpc":"2.0","id":1,"method":"initialize"}`
+		pty.WriteLine(payload)
+		_ = pty.ReadLine(ctx) // ignore echoed output
+		output := pty.ReadLine(ctx)
+		cancel()
+		<-cmdDone
+
+		// Ensure the initialize output is valid JSON
+		t.Logf("/initialize output: %s", output)
+		var initializeResponse map[string]interface{}
+		err := json.Unmarshal([]byte(output), &initializeResponse)
+		require.NoError(t, err)
+		require.Equal(t, "2.0", initializeResponse["jsonrpc"])
+		require.Equal(t, 1.0, initializeResponse["id"])
+		require.NotNil(t, initializeResponse["result"])
+	})
+
+	t.Run("NoCredentials", func(t *testing.T) {
+		t.Parallel()
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+		cancelCtx, cancel := context.WithCancel(ctx)
+		t.Cleanup(cancel)
+
+		client := coderdtest.New(t, nil)
+		inv, root := clitest.New(t, "exp", "mcp", "server")
+		inv = inv.WithContext(cancelCtx)
+
+		pty := ptytest.New(t)
+		inv.Stdin = pty.Input()
+		inv.Stdout = pty.Output()
+		clitest.SetupConfig(t, client, root)
+
+		err := inv.Run()
+		assert.ErrorContains(t, err, "your session has expired")
+	})
+}