refactor(agents): default model to gemini-3-flash-preview

Bump LlmAgent.DEFAULT_MODEL from gemini-2.5-flash to
gemini-3-flash-preview ahead of the 2026-10-16 earliest-shutdown date
for gemini-2.5-flash; see
https://ai.google.dev/gemini-api/docs/deprecations. Also updates the
docstring/example references in llm_agent_config.py and mcp_toolset.py
so the documented default tracks the actual default.

DEFAULT_LIVE_MODEL is unchanged — live mode has its own model family
(gemini-live-*) and is not affected by the gemini-2.5-flash
deprecation.

Change-Id: Icfbf1d60e75a4015847b56b24c9da62e52512073