[PY] Some unit tests are conditional upon the existence of `GEMINI_API_KEY`.

[yesudeep]

Describe the bug
Tests fail with:

=================================== short test summary info ===================================
FAILED plugins/google-genai/test/test_google_plugin.py::TestGoogleAIInit::test_init_raises_value_error_no_api_key - AssertionError: ValueError not raised
FAILED plugins/google-genai/test/test_google_plugin.py::TestGoogleAIInit::test_init_with_credentials - AssertionError: expected call not found.
Expected: Client(vertexai=False, api_key=None, credentials=<MagicMock spec='Credentials' id='NNNNNNNNN'>, debug_config=None, http_options=HttpOptions(base_url=None, api_version=None, headers={'x-goog-api-client': 'genkit-python/0.3.2', 'user-agent': 'genkit-python/0.3.2'}, timeout=None, client_args=None, async_client_args=None))
Actual: Client(vertexai=False, api_key='XXXXXXXXXXXXXXXXXXXXXX', credentials=<MagicMock spec='Credentials' id='NNNNNNNNN'>, debug_config=None, http_options=HttpOptions(base_url=None, api_version=None, headers={'x-goog-api-client': 'genkit-python/0.3.2', 'user-agent': 'genkit-python/0.3.2'}, timeout=None, client_args=None, async_client_args=None))

pytest introspection follows:

Kwargs:
assert {'vertexai': False, 'api_key': 'XXXXXXXXXXXXXXXXXXXXXX', 'credentials': <MagicMock spec='Credentials' id='NNNNNNNNN'>, 'debug_config': None, 'http_options': HttpOptions(base_url=None, api_version=None, headers={'x-goog-api-client': 'genkit-python/0.3.2', 'user-agent': 'genkit-python/0.3.2'}, timeout=None, client_args=None, async_client_args=None)} == {'vertexai': False, 'api_key': None, 'credentials': <MagicMock spec='Credentials' id='NNNNNNNNN'>, 'debug_config': None, 'http_options': HttpOptions(base_url=None, api_version=None, headers={'x-goog-api-client': 'genkit-python/0.3.2', 'user-agent': 'genkit-python/0.3.2'}, timeout=None, client_args=None, async_client_args=None)}

  Common items:
  {'credentials': <MagicMock spec='Credentials' id='NNNNNNNNN'>,
   'debug_config': None,
   'http_options': HttpOptions(base_url=None, api_version=None, headers={'x-goog-api-client': 'genkit-python/0.3.2', 'user-agent': 'genkit-python/0.3.2'}, timeout=None, client_args=None, async_client_args=None),
   'vertexai': False}
  Differing items:
  {'api_key': 'XXXXXXXXXXXXXXXXXXXXXX'} != {'api_key': None}

  Full diff:
    {
  -     'api_key': None,
  +     'api_key': 'XXXXXXXXXXXXXXXXXXXXXX',
        'credentials': <MagicMock spec='Credentials' id='NNNNNNNNN'>,
        'debug_config': None,
        'http_options': HttpOptions(base_url=None, api_version=None, headers={'x-goog-api-client': 'genkit-python/0.3.2', 'user-agent': 'genkit-python/0.3.2'}, timeout=None, client_args=None, async_client_args=None),
        'vertexai': False,
    }
========================= 2 failed, 346 passed, 13 warnings in 12.50s =========================

To Reproduce

1. Define the GEMINI_API_KEY environment variable with an API key value.

2. Tests fail.

3. Unset the GEMINI_API_KEY environment variable.

4. Tests pass.

Expected behavior
Tests should cover both cases (environment variable defined and undefined).

Screenshots
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Runtime (please complete the following information):

• OS: macOS
• Genkit Version HEAD

Python version

zsh❯ uv run python --version
Python 3.12.8

Additional context
Add any other context about the problem here.