Refactor test model creation code to shared utility

[google-labs-jules]

Extracted duplicated newGeminiTestClientConfig function into internal/testutil package as NewGeminiTestClientConfig to be shared across tests. This function configures the GenAI client for record and replay during testing. Updated tool/functiontool/function_test.go and model/gemini/gemini_test.go to use this shared utility.

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PR created automatically by Jules for task 299487248176627543 started by @sheing-google

[google-labs-jules]

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