feat(adk): export NewEventSenderToolWrapper for customizable tool event position

[shentongmartin]

Summary

Item	Details
Problem	eventSenderToolHandler was unexported and unconditionally injected, giving users no control over when tool-result AgentEvents fire in the middleware chain. The model side already had NewEventSenderModelWrapper for this.
Solution	Export NewEventSenderToolHandler() ChatModelAgentMiddleware so users place it in Handlers, controlling its position. Converge default and user paths onto the single eventSenderToolWrapper implementation.
Breaking	No — default behavior is preserved when the user does not supply the handler.

Key Insight

The model event sender used a clean interface type-assertion for detection (handler.(*eventSenderModelWrapper)), while the initial tool implementation used sync.Map + reflect.ValueOf().Pointer() because compose.ToolMiddleware is a struct, not an interface. Switching the tool event sender to a ChatModelAgentMiddleware (just like the model wrapper) eliminated this asymmetry — both now use the same detection pattern. The default path also converges: it creates a NewEventSenderToolHandler() and adapts it through handlersToToolMiddlewares, eliminating the old eventSenderToolHandler struct entirely.

What Changed

adk/wrappers.go

• Removed eventSenderToolHandler struct and its 4 compose-level methods (141 lines of dead code after convergence).
• Added eventSenderToolWrapper struct implementing ChatModelAgentMiddleware with 4 Wrap*ToolCall methods.
• Added NewEventSenderToolHandler() ChatModelAgentMiddleware constructor.
• Added hasUserEventSenderToolHandler() using type assertion (same pattern as hasUserEventSender for models).

adk/chatmodel.go

• Default path now creates NewEventSenderToolHandler() and converts it via handlersToToolMiddlewares(), then prepends as outermost — no direct compose.ToolMiddleware construction.
• Detection checks config.Handlers for *eventSenderToolWrapper via type assertion.
• Documentation updated with "Custom Tool Event Sender Position" section.

adk/wrappers_test.go

• 12 sub-tests covering all 4 tool types (Invokable, Streamable, EnhancedInvokable, EnhancedStreamable) x 3 scenarios (DefaultSendsEvent, UserConfiguredSkipsDefault, InnermostGetsOriginalOutput).
• Coverage: WrapInvokableToolCall 83%, WrapStreamableToolCall 90%, WrapEnhancedInvokableToolCall 81%, WrapEnhancedStreamableToolCall 88%, handlersToToolMiddlewares 81%.

Usage

agent, _ := adk.NewChatModelAgent(ctx, &adk.ChatModelAgentConfig{
    Handlers: []adk.ChatModelAgentMiddleware{
        myCustomHandler,                    // Outermost — modifies tool output
        adk.NewEventSenderToolHandler(),    // Innermost — events have original output
    },
})

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (alpha/09@b49a364). Learn more about missing BASE report.

Additional details and impacted files

@@             Coverage Diff             @@
##             alpha/09     #926   +/-   ##
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  Coverage            ?   81.46%           
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  Files               ?      158           
  Lines               ?    18902           
  Branches            ?        0           
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  Hits                ?    15399           
  Misses              ?     2367           
  Partials            ?     1136           

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