fix(adk): emit tool result AgentEvent after tool middlewares

[shentongmartin]

fix(adk): emit tool result AgentEvent after tool middlewares

Problem

Tool results emitted in AgentEvent were captured before middleware processing. When middlewares modify tool outputs (e.g., content filtering, format transformation), users received the raw results instead of the processed ones.

This happened because tool results were emitted via OnToolEnd callbacks, which fire at the component boundary—before the middleware chain processes the output.

Solution

Move tool result emission from callbacks into a collector middleware that sits at the outermost position of the middleware chain. This ensures it captures the final result after all other middlewares have processed it.

// Prepend collector middleware so it wraps all user middlewares
config.ToolCallMiddlewares = append(
    []compose.ToolMiddleware{newToolResultCollectorMiddleware()},
    config.ToolCallMiddlewares...,
)

The collector receives the tool result sender via context (set in graph's OnStartFn), and emits the result after next() returns.

Key Design Decisions

Why context instead of state for sender passing?

In interrupt/resume scenarios, OnStartFn is called before state is restored from checkpoint. Using context ensures the sender is available regardless of state restoration timing.

Scope

Both adk.ChatModelAgent and flow/agent/react.MessageFuture are fixed with the same pattern. Tests cover both Invoke and Stream paths, verifying that middleware-modified results are correctly emitted.

[codecov]

Codecov Report

❌ Patch coverage is 91.09589% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.28%. Comparing base (47ddc7e) to head (72a8ab5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
adk/chatmodel.go	83.72%	2 Missing and 5 partials ⚠️
flow/agent/react/react.go	87.87%	2 Missing and 2 partials ⚠️
adk/react.go	95.91%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #649      +/-   ##
==========================================
+ Coverage   80.23%   80.28%   +0.04%     
==========================================
  Files         124      124              
  Lines       11829    11904      +75     
==========================================
+ Hits         9491     9557      +66     
- Misses       1612     1616       +4     
- Partials      726      731       +5     

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