fix(go/googlegenai): Merge tools instead of replacing when using ai.WithTools()

[marcellmars]

I needed to combine a custom Genkit tool with recent FileSearch RAG feature. It fits naturally.

It didn’t work, and I eventually realized that ai.WithTools() was overwriting any tools provided via config.Tools instead of merging them. Silently.

The fix was simple replacing gcc.Tools = tools with gcc.Tools = mergeTools(append(gcc.Tools, tools...)) and the built-in tools (FileSearch, GoogleSearch, CodeExecution, Retrieval) remain intact when adding custom tools.

After making the change, I found out that the Gemini API currently doesn’t allow mixing built-in tools with custom (tools) function calling at all:

Error 400: Tool use with function calling is unsupported

Unfortunate, but hopefully temporary.

Once this pull request is merged:

• the SDK will report a clear API error instead of silently dropping tools
• the code will be ready the moment Google enables the natural fit of tools working together
• the Go SDK will match the JS SDK’s behavior, which already merges tools. Here's the test:

https://github.com/firebase/genkit/blob/f88c7bc7dfb73fd39aeffbdc99015f21c553c9ec/js/plugins/google-genai/tests/googleai/gemini_test.ts#L266

Testing

• Added TestToolMerging to verify the merge behavior
• Updated the live test to document and expect the API limitation