feat: add opt-in OAuth auth mode (auth_mode="oauth")

[karolzlot]

Adds a new auth_mode kwarg to run_gemini_cli_headless(...) to allow users authenticated via gemini auth login (OAuth / subscription flow) to use the wrapper without a GEMINI_API_KEY.

The default behavior (auth_mode="api_key") remains fully backward-compatible. The previous code always raised ValueError("GEMINI_API_KEY is missing."), which blocked users relying solely on OAuth credentials.

Key Changes

• gemini_cli_headless.py: Added auth_mode parameter. When set to "oauth", it skips the API key requirement and strips inherited GEMINI_API_KEY and GEMINI_CLI_HOME from the subprocess environment so the underlying CLI can resolve OAuth credentials natively.
• Testing: Added mock-based unit tests in tests/test_auth_mode.py that verify both authentication modes and isolation behaviors without requiring a real API key.
• Documentation: Updated README.md and docs/ with OAuth usage examples.
• Version: Bumped version to 5.1.0 in pyproject.toml.

Test plan

• pytest tests/test_auth_mode.py passes locally (keyless).
• pytest tests/test_quota_greedy_fix.py passes.
• Reviewer runs tests/integration/ with a real API key to confirm the default path is unchanged.
• Manual smoke test by an OAuth user.