Python: Fix GoogleAI validation: skip API key check when using Vertex AI

[veeceey]

Summary

Fixes #13483 - GoogleAIChatCompletion now correctly skips API key validation when use_vertexai=True.

Changes

• Updated API key validation in GoogleAIChatCompletion constructor to only require api_key when use_vertexai is False
• Added unit test to verify initialization succeeds with Vertex AI mode without an API key

Test Plan

• Added test_google_ai_chat_completion_init_with_vertexai_without_api_key to verify the fix
• Existing tests ensure API key is still required when use_vertexai=False
• Existing tests ensure project ID is required when use_vertexai=True

The validation logic now correctly differentiates between:

• Vertex AI mode (use_vertexai=True): Requires project_id, API key is optional (uses ADC)
• Standard mode (use_vertexai=False): Requires api_key