Fix auto chat rename for large histories

[ruizanthony]

Summary

Fixes automatic chat renaming when chat histories are large or contain structured/tool-call messages.

The previous implementation serialised the full chat history with history.output_text(), which can become very large and make auto rename slow or fail silently. It could also persist invalid utility-model responses such as JSON/tool-call payloads as chat titles.

Changes

• Build the rename prompt from a bounded set of recent messages via history.all_messages() when available.
• Ignore tool-call/tool-result payloads while extracting history text for chat titles.
• Normalize and validate model-generated names before saving them.
• Accept useful JSON wrappers such as {\"headline\": \"...\"} while rejecting tool-call JSON.
• Add a local fallback title generator, used especially for forced auto rename flows.
• Save the renamed context and notify the state monitor so the sidebar refreshes.
• Log auto-rename failures instead of swallowing them silently.

Verification

• PYTHONPATH=/tmp/a0-pr-auto-chat-rename /a0/venv/bin/python -m py_compile extensions/python/monologue_start/_60_rename_chat.py
• Smoke test for:
  • accepting normal chat-title strings;
  • rejecting obvious UI dumps;
  • accepting JSON headline wrappers;
  • rejecting tool-call JSON;
  • extracting bounded recent history;
  • forced fallback rename path.