feat(firebaseai): add responseJsonSchema to GenerationConfig

[gspencergoog]

Description

This adds the responseJsonSchema field to the GenerationConfig class, which enables more JSON schema features like $def and $ref along with other advanced JSON schema features that Gemini 2.5 supports.

See the Gemini docs for more information about what is supported.

Related Issues

• Fixes 🚀 [firebase_ai] Support for advanced Schema features with $ref and $def #17551

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Breaking Change

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[rodydavis]

LGTM

[cynthiajoan]

Thanks for the PR, would like to check if it has been tested under both vertexAI and googleAI backend.