feat: harden gemini provider path

[gay00ung]

Summary

This PR hardens the native Gemini GenerateContent API path for d2t.

Included changes

• add config coverage for provider = "gemini"
• verify the default Gemini resolution path through TOML loading
• add a native request smoke test for:
  • POST /v1beta/models/{model}:generateContent
  • x-goog-api-key
• add provider-specific usage docs for Gemini

Why this change

d2t already had Gemini support in code, but the provider path still needed clearer validation around:

• default config behavior
• request endpoint and auth expectations
• user-facing setup guidance

This PR makes the Gemini path more explicit and easier to trust.

Validation

• ./gradlew --no-daemon :apps:cli:test --tests dev.diff2test.android.cli.D2tConfigTest
• ./gradlew --no-daemon :modules:test-generator:test --tests dev.diff2test.android.testgenerator.OpenAiResponsesTestGeneratorTest

Notes

• this PR focuses on the native Gemini provider path only
• qwen/custom provider behavior is not changed here