feat(ai): add GeminiLanguageModel tool calling

[andrewheard]

Add function calling (tool calling) support to GeminiLanguageModel conforming to Apple's FoundationModels framework.

• Added the .toolCalling capability to GeminiLanguageModel.
• Translates the Foundation Models ToolDefinition into a Gemini Tool and FunctionDeclaration using parametersJsonSchema.
• Maps GenerationOptions.ToolCallingMode (.allowed, .required,
  .disallowed) to Gemini FunctionCallingConfig.Mode.
• Streams function calls in GeminiLanguageModelExecutor, forwarding JSON-encoded arguments or empty fallback "{}" (omitted by the backend on empty arguments) into channel actions.
• Translates Transcript tool calls and tool outputs into Gemini Contentparts, preserving thought signatures from reasoning models and flushing buffered reasoning thoughts cleanly.
• Unpacks structured tool responses and wrap scalar text tool outputs under the result field (since Foundation Models supports primitives as tool outputs).
• Support parameterless (void) tools with empty arguments across request schema translation, model response handling, and transcript history.
• Add unit tests across GeminiLanguageModel, GeminiRequestTranslator, and GeminiTranscriptTranslator.
• Add end-to-end integration tests covering single-turn, sequential, parallel, disallowed, parameterless, and reasoning tool calling across all supported backends.

#no-changelog

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