feat(go): improved tool interrupts API to be more ergonomic

[apascal07]

This PR introduces a new strongly-typed API for tool interrupts in Genkit Go, enabling human-in-the-loop workflows with better type safety and ergonomics.

New APIs

Triggering Interrupts

ai.InterruptWith[T](ctx, meta) - Interrupt tool execution with typed metadata:

return TransferOutput{}, ai.InterruptWith(ctx, TransferInterrupt{
    Reason:  "confirm_large",
    Amount:  input.Amount,
})

Handling Interrupts

ai.InterruptAs[T](part) - Extract typed metadata from an interrupt:

if meta, ok := ai.InterruptAs[TransferInterrupt](interrupt); ok {
    switch meta.Reason {
    case "confirm_large":
        // handle confirmation
    }
}

Resuming Interrupted Tools

Given a tool defined as:

myTool := genkit.DefineTool[TransferInput, TransferOutput](...)

ai.ToolDef[In, Out].RestartWith(part, opts...) - Re-execute tool with optional modifications:

// Simple restart (re-run with same input)
part, err := myTool.RestartWith(interrupt)

// Restart with replaced input (must be TransferInput)
part, err := myTool.RestartWith(interrupt, ai.WithNewInput(TransferInput{Amount: adjustedAmount}))

ai.ToolDef[In, Out].RespondWith(part, output, opts...) - Provide output directly without re-running:

// Output must be TransferOutput
part, err := myTool.RespondWith(interrupt, TransferOutput{Status: "cancelled", Message: "User declined"})

Tool Context Helpers

ai.ToolContext.IsResumed() - Check if tool is being restarted after interrupt:

if !ctx.IsResumed() && input.Amount > 100 {
    return TransferOutput{}, ai.InterruptWith(ctx, ...)
}

ai.OriginalInputAs[T](ctx) - Access original input when WithReplaceInput was used:

if orig, ok := ai.OriginalInputAs[TransferInput](ctx); ok {
    message = fmt.Sprintf("Adjusted from $%.2f", orig.Amount)
}

Deprecated APIs

Deprecated	Replacement
tool.Respond(part, output, opts)	tool.RespondWith(part, output, opts...)
tool.Restart(part, opts)	tool.RestartWith(part, opts...)

Key Improvements

1. Type Safety: Generic functions ensure interrupt metadata and tool inputs/outputs are properly typed at compile time
2. Error Handling: New RestartWith/RespondWith return errors instead of silently returning nil on invalid input
3. Validation: Tool name validation prevents accidentally using the wrong tool to respond to an interrupt
4. Functional Options: WithNewInput[In] and WithResumedMetadata[In] provide a cleaner API than struct-based options

Sample

See samples/intermediate-interrupts/ for a complete example demonstrating a payment agent with:

• Insufficient balance handling (offers adjusted amount via WithNewInput)
• Large transfer confirmation (simple restart after approval)
• Cancellation (direct response via RespondWith)

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated: Updated tool interrupt docs for Go to reflect new APIs. genkit-ai/docsite#205

[gemini-code-assist]

Summary of Changes

Hello @apascal07, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on refining the Go API for tool interrupts, making it more intuitive and robust for developers. It introduces a generic tool definition, provides type-safe utilities for managing tool execution flow after interruptions, and standardizes internal data conversions. These changes aim to streamline the development of applications that require interactive or conditional tool execution within generative AI workflows.

Highlights

• Improved Tool Interrupts API: The Go API for handling tool interrupts has been significantly refactored to be more ergonomic and type-safe. This includes moving interrupt-related functions to methods on ToolContext and introducing new functional options for RespondWith and RestartWith.
• Generic ToolDef Type: The tool struct has been replaced with a generic ToolDef[In, Out any] type, enhancing type safety and clarity for tool definitions and interactions.
• New Helper Functions for ToolContext: Several new helper methods have been added to ToolContext, such as InterruptWith, InterruptAs, IsResumed, ResumedValue, and OriginalInputAs, to simplify common patterns when dealing with tool interruptions and resumptions.
• Standardized JSON Conversions: New generic utility functions MapToStruct and StructToMap have been introduced in go/internal/base/json.go to standardize JSON conversions, replacing custom implementations in various plugins.
• Enhanced Model Referencing: Model referencing in Google Gen AI plugins and samples has been updated to use full model names (e.g., googleai/gemini-2.5-flash) for consistency.
• New Tool Interrupts Sample: A new sample (go/samples/intermediate-interrupts/main.go) has been added to demonstrate the practical application of the improved tool interrupts API, showcasing human-in-the-loop interactions for a payment agent.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and well-executed improvement to the tool interrupts API, making it more ergonomic and type-safe. The changes include a major refactoring of the tool definition, the addition of several convenient helper functions, comprehensive new tests, and a new sample to demonstrate the feature. Overall, the code quality is high. I've identified a couple of minor issues related to API design that could lead to inconsistencies or potential runtime errors, and I've provided suggestions to address them.