[Linux2010]

Problem

When cancellation overlaps an execution that finishes in INPUT_REQUIRED, the execution's event consumer can stop before it processes the cancellation events. This can happen either after the pipe closes or after a cancellation event is enqueued successfully but no consumer remains. CancelTask then reports that the task cannot be canceled even though the task is still interruptible.

This is the non-terminal case left open by the earlier closed-pipe fix in #521 (related to #518).

Change

• Track whether each cancellation event is read by the running execution, without blocking the cancellation writer on consumer progress.
• Once that execution finishes in a non-terminal state, process only the unread cancellation events against a fresh task snapshot. Do not invoke the agent's cancellation callback again.
• Preserve the existing handling for executions that reach a terminal state, and document the handoff in the concurrency model.

Validation

• go test ./...
• go test -race ./internal/eventpipe ./internal/taskexec ./a2asrv ./integrations/executor/a2aacp
• Repeated race tests cover the closed-pipe and unread-enqueued-event windows, a full pipe with no consumer, avoiding a cancel/execution deadlock, and not processing an event twice.
• Handler tests cover streaming ending at INPUT_REQUIRED, followed by successful CancelTask and a CANCELED GetTask; mixed consumed/unread artifacts and push delivery; and an execution that reaches COMPLETED before cancellation. They also check callback and cleanup counts.

The original stream ends when execution reaches INPUT_REQUIRED; the later cancellation result is returned by CancelTask and visible through GetTask.