fix(gemini): convert native tool dicts instead of silently dropping them

[JamMaster1999]

Description

_convert_tool_spec skips any dict whose type isn't "function", so a gemini-native tool dict like {"google_search": {}} silently vanishes from the request — no error, no search. The typed form of the same tool already passes through via BUILT_IN_TOOLS; the dict spelling is the one form that dies.

Non-function dicts now go through types.Tool.model_validate: any gemini built-in converts, and a dict the SDK schema rejects raises a typed InvalidRequestError. provider_name is threaded into the converter for the error message, as mozilla-ai#1300 does for _convert_tool_choice.

Verified live on gemini-2.5-flash with "What was the S&P 500's closing value last Friday?" and tools=[{"google_search": {}}]:

	result
before	tool silently dropped — answers 4997.58 from stale training data
after	grounded — "closing value last Friday, August 14, 2026, was 7,785.76"

Tests: a native dict reaches the request as a typed Tool; function and native tools mix in one call; unknown dicts raise. Existing direct-caller tests updated for the new argument.

PR Type

• 🐛 Bug Fix

Relevant issues

Same silent-drop family as mozilla-ai#1300.

Checklist

• I understand the code I am submitting.
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[JamMaster1999]

Reviewed and approved; landed on main as merge 68b65d6 (resolved a one-hunk conflict with the mozilla-ai#1300 carry locally — GitHub could not auto-merge).