Merge Gemini tool mode into caller-supplied ToolConfig instead of replacing it

[PratikDhanave]

What

When a tool mode is set, buildParams in provider/geminiprovider/agent.go assigned a brand-new genai.ToolConfig holding only the FunctionCallingConfig, replacing any ToolConfig the caller had already supplied through the GenerateContentConfig escape hatch. genai.ToolConfig also carries RetrievalConfig (Vertex grounding) and IncludeServerSideToolInvocations; those were silently dropped.

This mutates the existing ToolConfig in place instead: it only overrides FunctionCallingConfig, preserving the other caller-set fields.

Why

GenerateContentConfig is documented as the pass-through for custom genai parameters, and the config is already copied wholesale into cfg via *cfg = p. Overriding a single sub-field rather than clobbering the whole pointer matches how the rest of buildParams merges options (e.g. it appends to cfg.Tools rather than replacing it). It also mirrors the .NET/Python providers, where tool-mode selection sets the function-calling mode without discarding other tool-config settings a caller supplied for grounding/retrieval.

Testing

Added TestToolModeMergesCallerToolConfig to the canonical agent_test.go. It drives a run with a caller ToolConfig{RetrievalConfig: ...}, a function tool, and WithToolMode(required), then inspects the outgoing request: functionCallingConfig.mode == ANY is applied and retrievalConfig.languageCode is preserved. The test fails before this change (retrievalConfig dropped) and passes after. go build ./..., go vet, and go test ./provider/geminiprovider/... are green.

[Copilot]

Pull request overview

This PR fixes how the Gemini provider applies tool-mode selection so it no longer overwrites a caller-supplied genai.ToolConfig, preserving other ToolConfig fields (e.g., RetrievalConfig for Vertex grounding) while still enforcing the requested function-calling mode.

Changes:

• Update buildParams to merge tool-mode settings into an existing ToolConfig instead of replacing the whole struct.
• Add a regression test ensuring RetrievalConfig is preserved when WithToolMode(required) is applied.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
provider/geminiprovider/agent.go	Merges tool-mode function-calling config into existing ToolConfig instead of clobbering it.
provider/geminiprovider/agent_test.go	Adds a test to verify caller-supplied RetrievalConfig is preserved when tool mode is set.

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[PratikDhanave]

Good catch — fixed in 61f3eef: I now shallow-clone the caller-supplied ToolConfig struct before overriding FunctionCallingConfig, so we no longer mutate the caller's aliased pointer (from the shallow *cfg = p copy) while still preserving their other fields like RetrievalConfig.

[github-actions]

Parity Review: ✅ No Issues Found

This PR fixes a bug in the unexported buildParams method of the Gemini provider — no exported Go APIs are added or changed.

What changed: When a tool mode is set, the code now merges FunctionCallingConfig into any caller-supplied ToolConfig (shallow-cloning the struct first) instead of creating a brand-new ToolConfig that silently drops other fields like RetrievalConfig (Vertex grounding) and IncludeServerSideToolInvocations.

Cross-repo consistency: This change mirrors the behavior of the .NET and Python providers, where tool-mode selection sets the function-calling mode without discarding other tool-config settings a caller supplied for grounding/retrieval. The Go provider was diverging from upstream; this PR corrects that divergence.

Scope: Only unexported implementation code is touched (buildParams is unexported). No exported types, option structs, builder patterns, or user-visible behavior contracts are altered.

Conclusion: The parity-approved label already present is correct and should remain. No public-api-change label is warranted.

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