chore(ai): Add unit tests for Live API

[daymxn]

This PR adds unit tests for the Live API decoding/encoding. Since a lot of the API surface is covered by implicit Encodable (or alternatives) markers, this PR only really addresses the structs that have custom decoding or encoding logic.

Namely, this means VoiceConfig and BidiGenerateContentServerMessage both have had unit tests added. This basically just ensures they encode/decode correctly.

#no-changelog

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Code Review

This pull request adds valuable unit tests for the Live API's encoding and decoding logic, focusing on VoiceConfig and BidiGenerateContentServerMessage. The tests are well-structured and cover the primary use cases effectively. I've added one minor suggestion to enhance the test coverage for BidiGenerateContentServerMessage by testing a minimal payload to ensure optional fields are handled correctly. Overall, this is a solid contribution to the project's test suite.