fix(title): retry title generation without json_schema response_format when the provider rejects it

[ZLF0621]

Problem

Some OpenAI-compatible providers (notably DeepSeek) do not implement the json_schema response_format type and reject the request with HTTP 400 ("This response_format type is unavailable now"). Titling then fails silently and the session keeps the derived (un-upgraded) title. Reported in #83390.

Fix

Wrap the title-generation call and retry once without the json_schema response_format when the error mentions response_format or "unavailable". The prompt still demands JSON-only output, and the existing loose JSON scan in _extract_title_text falls back to first-line prose for anything non-compliant.

Validation

• Reproduced the HTTP 400 on DeepSeek before the change; after it, title generation succeeds via the fallback path (verified locally against a real DeepSeek endpoint).
• Full relay/title/clarify test set passes: 78 passed.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #82890: both add the same focused one-time schema-free retry for title generation after a response_format rejection. #82890 remains the earlier open implementation.

[teknium1]

Thanks @ZLF0621 for working on the json_schema retry. This landed on main through #89589 and #113966 (5cc8177), which covers the same symptom on the primary and fallback auxiliary paths and omits the field up front for providers known to reject it. Closing as superseded by the landed fix — the tracking issue (#83390 cluster) is closed with the same references.