Gemini models enable internal thinking/reasoning tokens by default.
When generate_title() called call_llm() with max_tokens=64, Gemini
consumed the entire 64-token budget on internal thought tokens,
truncating the JSON title response before it could complete. The
fallback prose extractor then picked up the opening fence (```json)
or a bare brace as the session title.
Two-part fix:
1. title_generator.py: Pass reasoning_config={"enabled": False} to
call_llm() so thinking is explicitly disabled for title generation.
2. chat_completions.py: In _build_gemini_thinking_config, when
reasoning is disabled (enabled=False or effort="none"), set
thinkingBudget: 0 on Gemini models that support it (2.5+ and 3.x).
includeThoughts: False only hides thought parts from the response
while the model still reasons internally and bills thought tokens
against maxOutputTokens. thinkingBudget: 0 truly disables thinking
so thought tokens do not consume the max_tokens budget.
Fixes#91927
