feat: add database tables and API routes for agentic chat feature (#17570)

Backend portion of experimental `AgenticChat` feature:
- Adds database tables for chats and chat messages
- Adds functionality to stream messages from LLM providers using
`kylecarbs/aisdk-go`
- Adds API routes with relevant functionality (list, create, update
chats, insert chat message)
- Adds experiment `codersdk.AgenticChat`

---------

Co-authored-by: Kyle Carberry <kyle@carberry.com>

coderd/httpmw/chat.go

@@ -0,0 +1,59 @@
+package httpmw
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/google/uuid"
+
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/httpapi"
+	"github.com/coder/coder/v2/codersdk"
+)
+
+type chatContextKey struct{}
+
+func ChatParam(r *http.Request) database.Chat {
+	chat, ok := r.Context().Value(chatContextKey{}).(database.Chat)
+	if !ok {
+		panic("developer error: chat param middleware not provided")
+	}
+	return chat
+}
+
+func ExtractChatParam(db database.Store) 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()
+			arg := chi.URLParam(r, "chat")
+			if arg == "" {
+				httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+					Message: "\"chat\" must be provided.",
+				})
+				return
+			}
+			chatID, err := uuid.Parse(arg)
+			if err != nil {
+				httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+					Message: "Invalid chat ID.",
+				})
+				return
+			}
+			chat, err := db.GetChatByID(ctx, chatID)
+			if httpapi.Is404Error(err) {
+				httpapi.ResourceNotFound(rw)
+				return
+			}
+			if err != nil {
+				httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+					Message: "Failed to get chat.",
+					Detail:  err.Error(),
+				})
+				return
+			}
+			ctx = context.WithValue(ctx, chatContextKey{}, chat)
+			next.ServeHTTP(rw, r.WithContext(ctx))
+		})
+	}
+}