feat: add experimental agents support (#22290)

feat: add AI chat system with agent tools and chat UI

Introduce the chatd subsystem and Agents UI for AI-powered chat
within Coder workspaces.

- Add chatd package with chat loop, message compaction, prompt
  management, and LLM provider integration (OpenAI, Anthropic)
- Add agent tools: create workspace, list/read templates, read/write/
  edit files, execute commands
- Add chat API endpoints with streaming, message editing, and
  durable reconnection
- Add database schema and migrations for chats, chat messages, chat
  providers, and chat model configs
- Add RBAC policies and dbauthz enforcement for chat resources
- Add Agents UI pages with conversation timeline, queued messages
  list, diff viewer, and model configuration panel
- Add comprehensive test coverage including coderd integration tests,
  chatd unit tests, and Storybook stories
- Gate feature behind experiments flag

---------

Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: Danielle Maywood <danielle@themaywoods.com>
Co-authored-by: Jeremy Ruppel <jeremy@coder.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

coderd/httpmw/requestid_test.go

@@ -1,11 +1,13 @@
 package httpmw_test
 
 import (
+	"context"
 	"net/http"
 	"net/http/httptest"
 	"testing"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/google/uuid"
 	"github.com/stretchr/testify/require"
 
 	"github.com/coder/coder/v2/coderd/httpmw"
@@ -31,3 +33,16 @@ func TestRequestID(t *testing.T) {
 	require.NotEmpty(t, res.Header.Get("X-Coder-Request-ID"))
 	require.NotEmpty(t, rw.Body.Bytes())
 }
+
+func TestRequestIDHelpers(t *testing.T) {
+	t.Parallel()
+
+	requestID := uuid.New()
+	ctx := httpmw.WithRequestID(context.Background(), requestID)
+	req := httptest.NewRequest(http.MethodGet, "/", nil).WithContext(ctx)
+
+	gotRequestID, ok := httpmw.RequestIDOptional(req)
+	require.True(t, ok)
+	require.Equal(t, requestID, gotRequestID)
+	require.Equal(t, requestID, httpmw.RequestID(req))
+}