Fixes #4836: Add gemini-2.5-flash model with correct pricing

[roomote]

Summary

This PR fixes issue #4836 by adding the missing model to the Gemini provider configuration with the correct pricing.

Changes

• Added model to the Gemini models configuration
• Set correct pricing according to Google's official documentation:
  • Input tokens: /bin/sh.30/M
  • Output tokens: .50/M
• Added support for prompt caching with appropriate cache pricing:
  • Cache reads: /bin/sh.075/M
  • Cache writes: .0/M
• Model configuration includes:
  • 65,535 max tokens
  • 1,048,576 context window
  • Image support enabled
  • Prompt caching support enabled

Testing

• All existing tests pass
• The model is now available for selection in the Google Gemini provider

References

• Fixes Gemini 2.5 Flash - incorrect pricing #4836
• Pricing source: https://ai.google.dev/gemini-api/docs/pricing#gemini-2.5-flash

---

Important

Adds gemini-2.5-flash model with updated pricing and prompt caching to Gemini provider configuration.

• Behavior:
  • Adds gemini-2.5-flash model to gemini.ts and vertex.ts with updated pricing.
  • Sets input price to 0.3 and output price to 2.5 for gemini-2.5-flash-preview-05-20 and gemini-2.5-flash-preview-04-17.
  • Enables prompt caching with cache read price 0.0375 and write price 1.0.
• Model Configuration:
  • maxTokens set to 65,535 and contextWindow to 1,048,576.
  • Supports images and prompt caching.
• Testing:
  • Model is now available for selection in Google Gemini provider.

^{This description was created by for 6675722. You can customize this summary. It will automatically update as commits are pushed.}

[sr-tream]

Pricing for -preview models remains the same on the Vertex AI page and in Google AI Studio. The price changed is only for stable release of gemini-2.5-flash

Set correct pricing according to Google's official documentation:

• Input tokens: /bin/sh.30/M
• Output tokens: .50/M

Added support for prompt caching with appropriate cache pricing:

• Cache reads: /bin/sh.075/M
• Cache writes: .0/M

What?

[daniel-lxs]

Closing, this PR changes the price for other flash unrelated models