fix(harness): acknowledge background task termination

[guslegend0510]

Summary

Fixes #2162.

Background task cancellation previously updated the persisted task state to
CANCELLED without providing authoritative acknowledgement that the underlying
local or remote execution had actually stopped.

This change introduces a typed cancellation contract that separates cancellation
request state from execution termination state.

Changes

• Add TaskCancellation with typed:
  • cancellation request status;
  • execution identity and execution kind;
  • termination status;
  • asynchronous termination signal;
  • caller-bounded awaitTermination(Duration) API.
• Preserve compatibility with the existing boolean TaskRepository.cancelTask(...)
  API.
• Track local execution lifecycle separately from CompletableFuture cancellation.
• Confirm local termination only after the supplier execution actually exits.
• Interrupt local execution cooperatively without treating interruption delivery as
  stop acknowledgement.
• Report adopted executions as detached when the repository cannot observe their
  underlying execution.
• Propagate remote cancellation failures through the typed termination result.
• Poll remote task status after the cancel request and acknowledge termination only
  after the remote protocol reports a terminal state.
• Make repeated cancellation requests idempotent and reuse the same termination
  signal.
• Update task_cancel output to distinguish request status from stop status.

Behavior

TaskStatus.CANCELLED continues to represent the durable repository state and
cancellation intent.

It no longer implies that the child execution has stopped. Callers that need to
perform workspace cleanup or parent shutdown can wait for the termination signal
with their own timeout.

Java does not provide a safe forced thread-stop mechanism. A local supplier that
ignores interruption therefore returns TIMED_OUT to bounded waiters until the
supplier actually exits.

Tests

Added coverage for:

• a local supplier that continues after interruption;
• cooperative local execution termination;
• caller-provided termination timeout;
• idempotent repeated cancellation;
• missing tasks;
• adopted/detached execution;
• remote cancellation confirmation;
• authoritative remote cancellation failure.

Validation completed successfully:

mvn -pl agentscope-harness -am test
mvn -pl agentscope-harness spotless:check

Branch: fix/2162-task-cancellation-ack
Commit: 5764d98

[codecov]

Codecov Report

❌ Patch coverage is 64.08163% with 88 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...s/agent/subagent/task/WorkspaceTaskRepository.java	51.12%	61 Missing and 26 partials ⚠️
.../harness/agent/subagent/task/TaskCancellation.java	97.95%	0 Missing and 1 partial ⚠️

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