fix(providers/google): omit function call ID for vertex ai to prevent error 400

[eswar-7116]

Fixes #265

Summary

• The Vertex AI REST endpoint rejects id as an unknown field on FunctionCall and FunctionResponse parts in multi-turn tool history. The Gemini Developer API accepts it so the fix is backend-aware rather than unconditional.
• toGooglePrompt now accepts an isVertexAI bool parameter and omits ID on both structs when targeting Vertex. The field is retained for the Developer API path keeping existing cassettes valid.

Tests

• I don't have GCP credentials to re-record the affected Vertex cassettes so I manually removed the id field from the request body sections of the vertex-gemini-2-5-flash and vertex-gemini-2-5-pro tool cassettes. Response sections are untouched. This keeps CI from hanging on the VCR matcher without requiring live Vertex credentials.

• Ran go test ./... -v -count=1 and all tests pass.

• I have read CONTRIBUTING.md.

• I have created a discussion that was approved by a maintainer (for new features).

[andreynering]

Some suggestions to make the code shorter and easier to understand.

[andreynering]

Suggested change

	}
	geminiPart := &genai.Part{
	FunctionCall: &genai.FunctionCall{
	ID: toolCall.ToolCallID,
	Name: toolCall.ToolName,
	Args: result,
	},
	}
	// Vertex breaks with a 400 if this field be present.
	if isVertexAI {
	geminiPart.FunctionCall.ID = ""
	}
	}

[andreynering]

Suggested change

	var functionResponse *genai.FunctionResponse
	if isVertexAI {
	functionResponse = &genai.FunctionResponse{
	Response: response,
	Name: toolCall.ToolName,
	}
	} else {
	functionResponse = &genai.FunctionResponse{
	ID: result.ToolCallID,
	Response: response,
	Name: toolCall.ToolName,
	},
	}
	}
	var functionResponse &genai.FunctionResponse{
	ID: result.ToolCallID,
	Response: response,
	Name: toolCall.ToolName,
	}
	// Vertex breaks with a 400 if this field be present.
	if isVertexAI {
	functionResponse.ID = ""
	}

[andreynering]

Suggested change

	var functionResponse *genai.FunctionResponse
	if isVertexAI {
	functionResponse = &genai.FunctionResponse{
	Response: response,
	Name: toolCall.ToolName,
	}
	} else {
	functionResponse = &genai.FunctionResponse{
	ID: result.ToolCallID,
	Response: response,
	Name: toolCall.ToolName,
	},
	}
	}
	var functionResponse &genai.FunctionResponse{
	ID: result.ToolCallID,
	Response: response,
	Name: toolCall.ToolName,
	}
	// Vertex breaks with a 400 if this field be present.
	if isVertexAI {
	functionResponse.ID = ""
	}

[andreynering]

Suggested change

	systemInstructions, content, warnings := toGooglePrompt(call.Prompt, g.providerOptions.backend == genai.BackendVertexAI)
	isVertexAI := g.providerOptions.backend == genai.BackendVertexAI
	systemInstructions, content, warnings := toGooglePrompt(call.Prompt, isVertexAI)

[eswar-7116]

Thanks for the review and the suggestions @andreynering !
I have applied the suggestions to make the code cleaner and easier to understand.