feat(chatd): persist last_error on chats table (#22436)

Adds a nullable `last_error` column to the `chats` table so error
reasons survive page reloads.

**Backend:**
- Migration adds `last_error TEXT` (nullable) to chats
- `UpdateChatStatus` writes the error reason when status transitions to
`error`, clears it (NULL) on recovery
- `convertChat` maps `sql.NullString` to `*string` in the SDK

**Frontend:**
- Sidebar falls back to `chat.last_error` when no stream error reason is
cached
- Chat detail page does the same for `persistedErrorReason`
- Fixtures updated for new required field

coderd/chatd/chatd.go

@@ -478,6 +478,7 @@ func (p *Server) EditMessage(
 			WorkerID:    uuid.NullUUID{},
 			StartedAt:   sql.NullTime{},
 			HeartbeatAt: sql.NullTime{},
+			LastError:   sql.NullString{},
 		})
 		if err != nil {
 			return xerrors.Errorf("set chat pending: %w", err)
@@ -739,6 +740,7 @@ func setChatPendingWithStore(
 		WorkerID:    uuid.NullUUID{},
 		StartedAt:   sql.NullTime{},
 		HeartbeatAt: sql.NullTime{},
+		LastError:   sql.NullString{},
 	})
 	if err != nil {
 		return database.Chat{}, xerrors.Errorf("set chat pending: %w", err)
@@ -753,6 +755,7 @@ func (p *Server) setChatWaiting(ctx context.Context, chatID uuid.UUID) (database
 		WorkerID:    uuid.NullUUID{},
 		StartedAt:   sql.NullTime{},
 		HeartbeatAt: sql.NullTime{},
+		LastError:   sql.NullString{},
 	})
 	if err != nil {
 		return database.Chat{}, err
@@ -810,6 +813,7 @@ func insertUserMessageAndSetPending(
 		WorkerID:    uuid.NullUUID{},
 		StartedAt:   sql.NullTime{},
 		HeartbeatAt: sql.NullTime{},
+		LastError:   sql.NullString{},
 	})
 	if err != nil {
 		return database.ChatMessage{}, database.Chat{}, xerrors.Errorf("set chat pending: %w", err)
@@ -1622,8 +1626,9 @@ func (p *Server) processChat(ctx context.Context, chat database.Chat) {
 		Valid: true,
 	})
 
-	// Determine the final status to set when we're done.
+	// Determine the final status and last error to set when we're done.
 	status := database.ChatStatusWaiting
+	lastError := ""
 	remainingQueuedMessages := []database.ChatQueuedMessage{}
 	shouldPublishQueueUpdate := false
 
@@ -1636,7 +1641,8 @@ func (p *Server) processChat(ctx context.Context, chat database.Chat) {
 		// Handle panics gracefully.
 		if r := recover(); r != nil {
 			logger.Error(cleanupCtx, "panic during chat processing", slog.F("panic", r))
-			p.publishError(chat.ID, panicFailureReason(r))
+			lastError = panicFailureReason(r)
+			p.publishError(chat.ID, lastError)
 			status = database.ChatStatusError
 		}
 
@@ -1707,6 +1713,7 @@ func (p *Server) processChat(ctx context.Context, chat database.Chat) {
 				WorkerID:    uuid.NullUUID{},
 				StartedAt:   sql.NullTime{},
 				HeartbeatAt: sql.NullTime{},
+				LastError:   sql.NullString{String: lastError, Valid: lastError != ""},
 			})
 			return updateErr
 		}, nil)
@@ -1746,7 +1753,8 @@ func (p *Server) processChat(ctx context.Context, chat database.Chat) {
 		}
 		logger.Error(ctx, "failed to process chat", slog.Error(err))
 		if reason, ok := processingFailureReason(err); ok {
-			p.publishError(chat.ID, reason)
+			lastError = reason
+			p.publishError(chat.ID, lastError)
 		}
 		status = database.ChatStatusError
 		return
@@ -2458,6 +2466,7 @@ func (p *Server) recoverStaleChats(ctx context.Context) {
 			WorkerID:    uuid.NullUUID{},
 			StartedAt:   sql.NullTime{},
 			HeartbeatAt: sql.NullTime{},
+			LastError:   sql.NullString{},
 		})
 		if err != nil {
 			p.logger.Error(ctx, "failed to recover stale chat",

coderd/chatd/chatd_test.go

@@ -577,6 +577,61 @@ func TestWaitingChatsAreNotRecoveredAsStale(t *testing.T) {
 		"waiting chat should not be modified by stale recovery")
 }
 
+func TestUpdateChatStatusPersistsLastError(t *testing.T) {
+	t.Parallel()
+
+	db, ps := dbtestutil.NewDB(t)
+	_ = newTestServer(t, db, ps, uuid.New())
+
+	ctx := testutil.Context(t, testutil.WaitLong)
+	user, model := seedChatDependencies(ctx, t, db)
+
+	chat, err := db.InsertChat(ctx, database.InsertChatParams{
+		OwnerID:           user.ID,
+		Title:             "error-persisted",
+		LastModelConfigID: model.ID,
+	})
+	require.NoError(t, err)
+
+	// Simulate a chat that failed with an error.
+	errorMessage := "stream response: status 500: internal server error"
+	chat, err = db.UpdateChatStatus(ctx, database.UpdateChatStatusParams{
+		ID:          chat.ID,
+		Status:      database.ChatStatusError,
+		WorkerID:    uuid.NullUUID{},
+		StartedAt:   sql.NullTime{},
+		HeartbeatAt: sql.NullTime{},
+		LastError:   sql.NullString{String: errorMessage, Valid: true},
+	})
+	require.NoError(t, err)
+	require.Equal(t, database.ChatStatusError, chat.Status)
+	require.Equal(t, sql.NullString{String: errorMessage, Valid: true}, chat.LastError)
+
+	// Verify the error is persisted when re-read from the database.
+	fromDB, err := db.GetChatByID(ctx, chat.ID)
+	require.NoError(t, err)
+	require.Equal(t, database.ChatStatusError, fromDB.Status)
+	require.Equal(t, sql.NullString{String: errorMessage, Valid: true}, fromDB.LastError)
+
+	// Verify the error is cleared when the chat transitions to a
+	// non-error status (e.g. pending after a retry).
+	chat, err = db.UpdateChatStatus(ctx, database.UpdateChatStatusParams{
+		ID:          chat.ID,
+		Status:      database.ChatStatusPending,
+		WorkerID:    uuid.NullUUID{},
+		StartedAt:   sql.NullTime{},
+		HeartbeatAt: sql.NullTime{},
+		LastError:   sql.NullString{},
+	})
+	require.NoError(t, err)
+	require.Equal(t, database.ChatStatusPending, chat.Status)
+	require.False(t, chat.LastError.Valid)
+
+	fromDB, err = db.GetChatByID(ctx, chat.ID)
+	require.NoError(t, err)
+	require.False(t, fromDB.LastError.Valid)
+}
+
 func newTestServer(
 	t *testing.T,
 	db database.Store,