feat: support gemini through ai sdk (#10671)

* add support for gemini in ai sdk

* support thought signature conversion

* capture extra_content in assistasnt tool calls

* fix: update vercelStreamConverter tests for tool-call chunk emission

The tool-call case now returns a chunk (for Gemini thoughtSignature
support) instead of null, so tests need to expect the new behavior.

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Co-authored-by: Nate Sesti <33237525+sestinj@users.noreply.github.com>
Co-authored-by: Dallin Romney <dallinromney@gmail.com>