feat: add gemini_tools field to GeminiModel with validation and tests (#1050)

Add support for Gemini-specific tools like GoogleSearch and CodeExecution, with validation to prevent FunctionDeclarations.

Due to the fundamental differences in how Gemini's built-in tools operate (server-side execution without explicit tool call/result blocks), we don't implement history tracking for gemini_tools - that would require additional design work and a longer discussion on how to normalize this across all model providers. 

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Mackenzie Zastrow <zastrowm@users.noreply.github.com>