fix(gemini): preserve google search grounding tools

[YanzheL]

Summary

• preserve Google Search / web search tools when the Antigravity Claude→Gemini transformer also forwards normal function tools
• preserve Google Search / web search tools in the Gemini Claude-messages compatibility transformer and normalize direct AI Studio requests to the REST-facing google_search key
• add regression coverage for mixed tool payloads, including transform-level assertions and posted-body verification for the AI Studio path

Validation

• go test ./internal/pkg/antigravity -run 'TestBuildTools_|TestBuildParts_|TestTransformClaudeToGeminiWithOptions_'
• go test ./internal/service -run 'TestConvertClaudeToolsToGeminiTools_|TestConvertClaudeMessagesToGeminiGenerateContent_|TestEnsureGeminiFunctionCallThoughtSignatures_|TestGeminiMessagesCompatServiceForward_'
• go test ./...
• go build ./cmd/server

Closes #1161