fix: accumulate streamed function call arguments

[CREATEDCHARACTR]

Summary

• accumulate streamed function-call partial arguments into final Args values during GenerateContentStream and Live Session.Receive
• merge completed streamed function calls into chat history while preserving ordered completed calls
• add strict JSONPath mutation, conflict checks, array growth, null handling, and WillContinue cleanup helpers

Verification

• go test --mode=unit -v ./...

Note: this PR is opened from local RM3 review evidence. It does not claim Google approval or merge.

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