fix: avoid 500 when generating chat title from rename dialog and remove unused endpoint - #28306

Clicking Generate in the rename dialog (or Regenerate) runs manual chat
title generation under an internal deadline. When the model call times
out, the error ("context deadline exceeded") bubbled up to the propose
and regenerate handlers and fell through to a raw 500 that leaked the
wrapped error chain.

Translate context.DeadlineExceeded to 504 with a friendly retry message
and context.Canceled to 499 (client closed request) in both the
propose-title and regenerate-title handlers, matching by errors.Is so
the wrapped deadline error is detected. Genuine, unrelated failures keep
their existing 500 surface.

Generated by Coder Agents.

…tion

Manual chat title generation (rename dialog Generate and Regenerate)
previously ran a single model under a 30s deadline. A slow provider hit
the deadline and failed the request, which the handler now surfaces as a
504.

Walk the enabled preferred short-text models the user has credentials
for, then the chat's own model, bounded by an overall 90s budget with a
30s per-attempt deadline. Fall through to the next candidate only on
per-attempt timeout or chatretry-classified transient errors; stop on
non-retryable errors (auth, config) so real failures still surface.

When the context is canceled after a candidate has already failed, the
walker surfaces ctx.Err() instead of the stale candidate error, so the
handler maps caller cancellation to 499 and overall-budget expiry to 504
rather than a stale 500.

Fallback models resolve lazily so a run that succeeds on the first
candidate never constructs the others. Replaces the single-model
resolveManualTitleModel (and the now-unused
selectPreferredConfiguredShortTextModelConfig) with
resolveManualTitleCandidates.

Generated by Coder Agents.

…llback

Address two review findings:
- Map manual title errors to 504 only when a title deadline actually
  expired, via the chatd.ErrManualTitleTimedOut sentinel, so provider
  failures that merely wrap a transport deadline keep the 500 surface.
- Append the chat's own model as the final walk candidate when preferred
  short-text models exist, so the walk can still succeed when every
  preferred candidate fails. It skips itself when it duplicates an
  already-attempted preferred config.

The frontend only uses POST /chats/{chat}/title/propose (rename dialog);
nothing calls /title/regenerate anymore. Remove the endpoint, its
codersdk client method, the chatd persist/broadcast path it alone used
(persistManualTitle, regenerateChatTitleWithStore), and its tests. Port
the paste-only-chat coverage to the propose suite and add the missing
swagger annotations for the propose endpoint so it appears in the API
reference.

…ation

Restore the single-model manual title path (resolveManualTitleModel and
selectPreferredConfiguredShortTextModelConfig) and delete the candidate
walk (titlewalk.go). Keep the 30s generation deadline and the
ErrManualTitleTimedOut sentinel so the handler still maps genuine title
timeouts to a friendly 504 and caller cancellation to 499.

Generated by Coder Agents.