Establishes the integration testing foundation and parameterized test
suites for `GeminiLanguageModel`, enabling fast local execution via
`swift test` and automated CI verification on iOS Simulator.
Key Changes:
- `IntegrationTestingBackend`: Defines canonical backend configurations
(`.developerAPI`, `.firebaseAILogicDeveloperAPI`, and
`.firebaseAILogicAgentPlatform`), dynamically resolving endpoints,
model resource paths, and auth headers from available credentials.
- `IntegrationTestEnvironment`: Centralizes credential discovery
(`GEMINI_API_KEY`, `GOOGLE_API_KEY`, and `FIREBASE_PLIST_PATH`) into
an immutable, thread-safe configuration struct with cached process
access to avoid redundant disk I/O.
- `AppCheckDebugClient`: Implements a lightweight client to exchange
debug tokens for App Check tokens without Keychain dependencies,
deduplicating token requests across test runs with an in-memory cache.
- Parameterized Tests: Adds `BasicContentGenerationIntegrationTests`
running against all available backends, tagged with `@Tag.integration`
and gated by condition traits to cleanly skip when credentials are
missing.
- CI Workflow: Adds `.github/workflows/sdk.geminilanguagemodel.yml` to
run integration tests against Firebase AI Logic on iOS Simulator
using decrypted repo secrets.
