fix(a2asrv): skip TaskID mismatch check when message has no task reference (fixes #350)

[kuangmi-bit]

What

Fix loadExecutionContext to not reject messages that have no TaskID set.

Why

In cluster mode with retries enabled, the function reaches the TaskID mismatch check at line 195 even for messages that never referenced any task (message.TaskID == ""). Since an empty string can never equal a real tid, this condition was always true, causing a false "bug: message task id different from executor task id" error.

Fix

Add an empty-string guard:

// Before
if message.TaskID != tid {

// After  
if message.TaskID != "" && message.TaskID != tid {

Messages with no task reference are now accepted (they will be handled by the subsequent context creation / history replay logic), while messages that DO reference a task but the wrong one are still correctly rejected.

Closes #350

[gemini-code-assist]

Code Review

This pull request updates the task ID validation in a2asrv/agentexec.go to allow an empty message.TaskID. The review feedback suggests improving the error handling for mismatched task IDs by wrapping the error with a2a.ErrInvalidParams and including the mismatched IDs in the error message, which prevents misclassifying client-side errors as internal server bugs.

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[gemini-code-assist]

To improve debugging and align with the error handling pattern used for context ID mismatches (on line 203), consider wrapping this error with a2a.ErrInvalidParams and including the mismatched IDs in the error message. This avoids misclassifying a client-side parameter mismatch as an internal server bug.

Suggested change

	if message.TaskID != "" && message.TaskID != tid {
	return nil, fmt.Errorf("bug: message task id different from executor task id")
	}
	if message.TaskID != "" && message.TaskID != tid {
	return nil, fmt.Errorf("message task id %q different from executor task id %q: %w", message.TaskID, tid, a2a.ErrInvalidParams)
	}

[nahapetyan-serob]

Thanks for tracking this down and preparing the fix, do you mind adding a test?

[kuangmi-bit]

Added regression test TestLoadExecutionContext_EmptyTaskIDWithRetry — constructs a factory with taskRetrySupported: true, seeds the task store with an existing task, and verifies loadExecutionContext accepts a message with empty TaskID. 38 packages pass with -race.