Fixes#18282
This PR fixes two issues with Gemini context window error handling:
1. **Pattern matching for Gemini 2.0 Flash**: The previous pattern
'input token count exceeds the maximum number of tokens allowed'
doesn't match Gemini 2.0 Flash errors which include dynamic token
counts like '(2800010)' in the message. Split into shorter patterns
that work with both formats.
2. **Add context window check to Gemini block**: The
is_error_str_context_window_exceeded() check was only called for
OpenAI-compatible providers, not for Gemini/Vertex AI. Added the
check to the Gemini-specific error handling block.
Test cases added for both Gemini 2.0 Flash and 2.5/3 error formats.