Python: fix(google-ai): skip api_key check when use_vertexai is True

[giulio-leone]

Summary

Fixes #13483

Motivation and Context

When use_vertexai=True, users authenticate via Application Default Credentials (ADC) and don't need an API key. However, the initialization check for api_key runs unconditionally, blocking Vertex AI users with:

ServiceInitializationError: The API key is required when use_vertexai is False.

Description

Guard the api_key validation with not google_ai_settings.use_vertexai in all three Google AI service classes:

• GoogleAIChatCompletion
• GoogleAITextCompletion
• GoogleAITextEmbedding

The error message already reads "when use_vertexai is False", so the condition now matches the intent.

Before / After

# Before — fails even with use_vertexai=True
GoogleAIChatCompletion(
    use_vertexai=True,
    cloud_project_id="my-project",
    cloud_region="us-central1"
)
# → ServiceInitializationError: The API key is required when use_vertexai is False.

# After — works correctly
GoogleAIChatCompletion(
    use_vertexai=True,
    cloud_project_id="my-project",
    cloud_region="us-central1"
)
# → Initializes successfully, uses ADC for authentication

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines
• The code follows the SK coding standards

[giulio-leone]

@microsoft-github-policy-service agree

[giulio-leone]

Friendly ping — CI is green and this is ready for review. Happy to address any feedback. Thanks!