Implemented `GeminiLanguageModel`, providing an adapter for Google's
Gemini models conforming to Apple's `FoundationModels.LanguageModel`
protocol. This PR includes text to text generation capabilities; other
modalities and Gemini features will be handled in follow-up PRs.
**Note:** The `GeminiLanguageModel` implementation is guarded by the
environment variable `GEMINI_LANGUAGE_MODEL`. By default none of the
`GeminiLanguageModel` implementation is included.
Key additions:
- `GeminiLanguageModel`: Implements `LanguageModel` protocol with
reasoning capabilities, configuring execution via `ModelResource`
and `EndpointConfiguration`.
- `GeminiLanguageModelExecutor`: Converts prompt instructions and
history into Gemini streaming requests, yielding responses as
`LanguageModelSession.ResponseStream`.
- `GeminiTranscriptTranslator`: Translates Apple `Transcript` entries
and segments into Gemini `Content` structures while preserving thought
signatures and validating supported transcript features.
- `GeminiLanguageModel.Error` and `GeminiErrorMapper`: Translates Gemini
API, HTTP, and network errors into standard `LanguageModelError` cases
(`rateLimited`, `timeout`, `guardrailViolation`, `refusal`, and
`unsupportedLanguageOrLocale`) or `GeminiLanguageModel.Error` cases
(`serviceUnavailable`, `networkFailure`, `modelNotFound`, `apiError`).
The `APIError` payload is designed for seamless forward compatibility
with both GenerateContent and the upcoming Interactions API.
- Testing infrastructure: Adds `ConditionTrait.requireFoundationModels`
to conditionally run Foundation Models tests only on supported
platforms (macOS 27+, iOS 27+, watchOS 27+, visionOS 27+), along with
shared test constants, mock fixtures, and comprehensive unit and
integration test suites.
