feat: implement GeminiLanguageModel for FoundationModels

[andrewheard]

Introduces GeminiLanguageModel to allow access to Gemini models through Apple's Foundation Models framework (LanguageModel protocol).

• Adds GeminiLanguageModelExecutor to translate model session requests into Gemini REST API calls via GeminiAPIClient.
• Adds GeminiTranscriptTranslator to handle Apple transcript formatting.
• Introduces BackendConfiguration to encapsulate backend URL routing.
• Secures the apiKey convenience initializer behind a Swift Package trait (DirectGeminiDeveloperAPIAccess) to prevent hardcoded keys in client apps from Firebase AI Logic.
• Includes comprehensive unit tests and mock protocols for the executor.

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