feat(go): add support for multi-part tools

[apascal07]

New Multipart Tool APIs:

// Define a multipart tool that returns output + content parts
imageGen := genkit.DefineMultipartTool(g, "generateImage", "Generates an image",
    func(ctx *ai.ToolContext, input ImageInput) (*ai.MultipartToolResponse, error) {
        imageBytes := generateImage(input.Prompt)
        return &ai.MultipartToolResponse{
            Output: map[string]any{"status": "success"},
            Content: []*ai.Part{
                ai.NewMediaPart("image/png", base64.StdEncoding.EncodeToString(imageBytes)),
            },
        }, nil
    },
)

Updated Tool Definition with Options:

DefineToolWithInputSchema and NewToolWithInputSchema are now deprecated. Use DefineTool/NewTool with ai.WithInputSchema() instead:

// Before (deprecated)
tool := genkit.DefineToolWithInputSchema(g, "search", "Search tool", inputSchema, fn)

// After
tool := genkit.DefineTool(g, "search", "Search tool", fn, ai.WithInputSchema(inputSchema))

Key Changes:

• MultipartToolResponse: New type with Output (any) and Content ([]*Part) fields
• DefineMultipartTool / NewMultipartTool: Create tools that return rich content
• WithInputSchema option: New options pattern for custom input schemas that is shared with DefinePrompt
• Deprecated: DefineToolWithInputSchema and NewToolWithInputSchema
• Tool.RunRawMultipart(): New method to get full multipart response from any tool

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)