fix(agent): surface empty assistant answers instead of silent stops (#1830)

The support chat showed 'Tänker' and then went quiet with no answer and
no error. Root cause: the 2026-08-21 RIP-3 cutover moved general.help to
the single-call POST /api/agent/ask, whose 1500-token default cap made
stop_reason max_tokens routine on tool-loop turns. The empty answer then
passed unlogged through the service, the route answered 200 with an
empty string, and the console appended an invisible empty bubble.

Fixes, single-call path:
- ask-service: default maxTokens 1500 -> 5400 (the streaming chat's
  reply ceiling); an empty final answer now logs model + usage and
  throws the typed EmptyModelAnswerError instead of passing through.
- /api/agent/ask: maxDuration 300, logger, empty answer maps to 502
  with 'Assistenten gav inget svar. Försök igen.'; an empty assistant
  turn is never persisted (the question stays, so retry works).
- AskConsole: a 200 with an empty answer shows the error box instead of
  appending an invisible bubble.
- anthropic-family: serialized tool results are bounded at 40000 chars
  (mirrors run-turn) so one big read cannot eat the output budget; the
  step-exhausted fallback keeps tools declared with tool_choice none,
  because replaying tool_use/tool_result without tools is an API 400.

Fixes, streaming path (same silent class):
- /api/agent/invoke: maxDuration 300 so deep thinking turns are not
  killed mid-stream at the platform default cap.
- run-turn: stop_reason max_tokens with no visible text emits an error
  event, not a bare turn_complete.
- AgentChat: an NDJSON stream that ends without turn_complete or error
  (and was not aborted) shows 'Anslutningen bröts innan svaret blev
  klart. Försök igen.'

The lib/ai request-shape tests were updated deliberately for the
fallback change; general.help stays on the single-call runtime
(founder decision, not reverted).


Claude-Session: https://claude.ai/code/session_01SyDuePXxUFowaPBKpAv8SF

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/api/agent/invoke/route.ts

@@ -17,6 +17,13 @@ import { getErrorMessage as getUserErrorMessage } from '@/lib/errors/get-error-m
 // agent tool registry which is populated by the mcp-server extension at load.
 ensureInitialized()
 
+// Deep turns (thinking at high/xhigh effort, up to 12 tool iterations, a
+// 15-27k-token prompt prefix) can outlive the plan default duration cap; a
+// function killed mid-stream closes the NDJSON stream cleanly, which the chat
+// used to render as silent success. 300 s matches the other long-running model
+// surfaces (app/api/receipt-hunt/run, app/api/agent/ask).
+export const maxDuration = 300
+
 // Hard cap on the per-turn user input. Generous for a chat composer (about
 // 5k words / 20 pages) but bounds Bedrock token cost if the rate limiter is
 // ever fail-open and a client floods large payloads.