Today, langfun's Gemini base class hardcodes toolConfig.functionCallingConfig.mode='NONE',

[copybara-service]

Today, langfun's Gemini base class hardcodes toolConfig.functionCallingConfig.mode='NONE',
unconditionally disabling function calling for ALL Gemini subclasses including
gemini-3.1-pro-preview (both Vertex AI and Google GenAI backends). Callers who want
function calling must pass BOTH extras['tools'] AND extras['toolConfig'] — easy to
forget, and inconsistent with langfun's other 'just-works' Gemini features
(e.g. implicit prompt caching).

This CL: in Gemini.request(), if sampling_options.extras['tools'] is non-empty,
default mode to 'AUTO' instead of 'NONE'. Callers can still override by passing
extras['toolConfig'] explicitly (the existing extras-merge runs after this default).
When no tools are passed, today's wire payload is preserved bit-for-bit (mode='NONE').

Design notes:

• No new API surface (no new LMSamplingOptions field, no new kwarg, no new method).
  Reuses the existing extras: dict[str, Any] escape hatch.
• Implicit / default-on, mirroring the implicit prompt caching precedent.
• Backward compatible: text-only callers (no tools in extras) see identical wire
  payload to today.

Tests:

• gemini_test.py: 8 new/updated unit tests covering legacy preservation, AUTO
  flip on tools, empty-list adversarial, caller override via extras['toolConfig'],
  zero-parameter tool, deeply nested schema, oversized (128) tool list,
  prompt-injection-style request integrity.
• vertexai_test.py: VertexAIGemini31ProPreview subclass smoke test.
• google_genai_test.py: Gemini31ProPreview subclass smoke test.

RED→GREEN evidence:
RED pre-change failure summary:

• gemini_test.py: FAILED (failures=5)
• google_genai_test.py: FAILED (failures=1)
• vertexai_test.py: FAILED (failures=1)
  All failed with: AssertionError: {'functionCallingConfig': {'mode': 'NONE'}} != {'functionCallingConfig': {'mode': 'AUTO'}}

GREEN post-change pass summary:

• gemini_test.py: PASSED
• google_genai_test.py: PASSED
• vertexai_test.py: PASSED

Live integration testing (deferred to follow-up CL): no live-test infrastructure
currently exists under //third_party/py/langfun/core/llms/ (no *_live_test.py,
no GCP project ID convention, no BUILD gating tags). Establishing that pattern
requires owner input on the project ID and OSS-mirror posture for
langfun.core.google.auth.gcp_corp_access (LINT.IfChange/copy.bara.sky).
Tracked separately.