fix(langchain): allow Gemini 3 models to use ProviderStrategy with tools

[Théo Deschamps (thdesc)]

Fixes #34463

Summary

Passing a raw Pydantic model to create_agent(..., response_format=MyModel) defaults to AutoStrategy, which checks _supports_provider_strategy before deciding between ProviderStrategy and ToolStrategy. For Gemini models the helper currently returns False whenever tools are present, so Gemini 3 models end up using ToolStrategy even though they support provider-specific structured output with tools (link). Explicitly passing ProviderStrategy(MyModel) works; only the AutoStrategy detection path is affected.

Impact

• Gemini 3 users invoking create_agent(..., response_format=MyModel) can’t access Gemini’s native structured output when tools are bound.
• Agents fall back to ToolStrategy.
• Users have to know to wrap their schema in ProviderStrategy manually, defeating the convenience of AutoStrategy.

Expected Behavior

• _supports_provider_strategy should return True for Gemini 3 models when tools are present, so AutoStrategy upgrades to ProviderStrategy automatically.
• Gemini versions prior to 3 (and the gemini-flash(-lite)-latest aliases until they point to 3-series backends) should continue to fall back to ToolStrategy.

Additional Context

• Logic resides in libs/langchain_v1/langchain/agents/factory.py::_supports_provider_strategy.
• Tests in libs/langchain_v1/tests/unit_tests/agents/test_response_format.py cover strategy selection, but there’s currently no case ensuring Gemini 3 + AutoStrategy succeeds.
• Explicit response_format=ProviderStrategy(...) is a temporary workaround but shouldn’t be required.

[Théo Deschamps (thdesc)]

This should fixe this issue #34463

[ccurme (ccurme)]

Thanks!