fix(title): fall back to json_object response_format on provider rejection

[defgn]

Problem

Two-stage session titling (instant derived title, then an LLM "upgrade" to a short summary) fails for providers that reject the strict json_schema response_format. DeepSeek V4 returns HTTP 400 ("This response_format type is unavailable now"), the title task returns None, and the raw derived title ([Display Name] first words…) sticks forever.

Root cause

generate_title sends extra_body={"response_format": _TITLE_RESPONSE_FORMAT} (json_schema) unconditionally. Some providers only support json_object. Because callers treat a None title as "keep the derived title", the failure is silent — no error surfaces to the user.

Fix

Retry the title call with {"type": "json_object"} when the json_schema call raises. The prompt already demands a single JSON object and _extract_title_text's loose scan tolerates non-compliant output, so the looser hint is safe.

Verification

• Live deployment with DeepSeek V4 as the title provider: before → threads stuck with [Name] <first words…>; after → clean 3–7 word summaries land ~3s after thread creation.
• The json_schema path is still attempted first; the fallback only fires on provider rejection and is logged at warning level.
• Models that accept json_schema (e.g. openai-api) are unaffected.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #82073. Both implement the same json_schema-to-json_object retry for title generation; #82073 is the safer tested canonical version because it only retries a response-format rejection.