feat(google): Agentic Video Understanding support

[Allen Firstenberg (afirstenberg)]

Adds support for Agentic Video Understanding (see https://blog.google/innovation-and-ai/models-and-research/gemini-models/introducing-agentic-video-in-gemini/)

• Adds support for mediaProcessing parameter in a video content block
• Changes API version for GCP / GEAP to "v1beta1", which is necessary for mediaProcessing
• Adds tests for same, specific to models that can accept it
• Adds gemini-3.8-flash to test suite
• Updates to latest API
• Changes how we save the OpenAPI spec for the API (sort parameters alphabetically so future diffs will be useful).

TODO:

• Check merge conflict
• Make sure thought signatures that are necessary are sent back
• Double-check the tests to make sure they are valid

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Latest commit: 03ea5b1

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[Allen Firstenberg (afirstenberg)]

This should be removed. Despite being valid in the API, it shouldn't be valid for a user to set it.