feat: show agent turn summary in agents sidebar (#24942)

Persists the agent-generated turn-end summary on `chats` and shows it as
the Agents sidebar subtitle when present, falling back to the model
name. Errors still take precedence.

> Mux is acting on Mike's behalf.

## What changes

**Storage.** New nullable `last_turn_summary` column on `chats`
(migration `000486`). New `UpdateChatLastTurnSummary` query normalizes
blank/whitespace input to `NULL`, preserves `updated_at` (so the chat
does not jump to the top of the sidebar on summary writes), and uses an
`expected_updated_at` stale-write guard so an older async summary cannot
overwrite a newer turn.

**Backend.** `coderd/x/chatd/chatd.go` decouples summary generation from
webpush. Generated summaries persist for completed parent turns even
when webpush is unconfigured or has no subscriptions. The same generated
text is reused as the webpush body when webpush is configured, so the
summary model is not called twice. Generic fallback push text is no
longer persisted; it clears any stale summary instead.
Error/interrupt/pending-action terminal paths clear `last_turn_summary`
for the latest turn.

**Frontend.** `AgentsSidebar.tsx` subtitle priority is now `errorReason
|| lastTurnSummary || modelName`, normalized via the existing
`asNonEmptyString` helper from `blockUtils.ts`.

## Tests

- `TestUpdateChatLastTurnSummary` (database): success,
whitespace-to-NULL, stale guard rejects, `updated_at` preserved.
- `TestUpdateLastTurnSummaryRejectsStaleWrites` (chatd internal): direct
stale-`expected_updated_at` test.
- `TestSuccessfulChatPersistsTurnSummaryWithoutWebPush`: persistence
works without webpush subscriptions.
- `TestSuccessfulChatSendsWebPushWithSummary`: same generated text
drives both DB and push body.
-
`TestSuccessfulChatSendsWebPushFallbackWithoutSummaryForEmptyAssistantText`:
fallback text is not persisted.
- `TestErroredChatClearsLastTurnSummaryAndSendsWebPush`: error path
clears the field.
- `TestInterruptChatDoesNotSendWebPushNotification`: interrupt path
clears the field, no push fires.
- `AgentsSidebar.test.tsx`: subtitle priority for summary-present,
error-wins, no-summary fallback, whitespace fallback.
- `AgentsSidebar.stories.tsx`: `ChatWithTurnSummary` and
`ChatWithTurnSummaryAndError`.

## Notes

- No backfill. Existing chats keep showing the model name until their
next turn completes.
- Parent chats only in this iteration; the field is rendered on any
`Chat` if a future change extends generation to children.
- Decoupling generation from webpush adds quickgen model calls for
completed parent turns that previously skipped generation when no
subscriptions existed. Existing parent-only, assistant-text-present,
`PushSummaryModel` configured, and bounded-timeout gates keep this
behavior bounded.

coderd/database/dump.sql

@@ -1451,6 +1451,7 @@ CREATE TABLE chats (
     organization_id uuid NOT NULL,
     plan_mode chat_plan_mode,
     client_type chat_client_type DEFAULT 'api'::chat_client_type NOT NULL,
+    last_turn_summary text,
     CONSTRAINT chats_pin_order_archived_check CHECK (((pin_order = 0) OR (archived = false))),
     CONSTRAINT chats_pin_order_parent_check CHECK (((pin_order = 0) OR (parent_chat_id IS NULL)))
 );