Verify completed-task cancel race (#140) is resolved on main; close superseded PR #180

### Summary

Verify the completed-task cancel race (#140) is actually resolved on current `main`, then close the now-superseded draft PR #180.

### Background

Draft PR #180 fixes the race in `tests/mandatory/protocol/test_tasks_cancel_method.py`, but that file no longer exists on `main` — the `tests/mandatory/` tree was restructured into `tests/compatibility/` (commit `2bb20c2`), which is why the PR is `CONFLICTING`. The race appears already handled by the replacement coverage in `tests/compatibility/core_operations/test_task_lifecycle.py`:

- `TestCancelTask.test_cancel_task_returns_updated_state` (CORE-CANCEL-001) tolerates the terminal-state race.
- `TestCancelTask.test_cancel_terminal_task_returns_error` (CORE-CANCEL-002) asserts `-32002` TaskNotCancelableError on terminal tasks.

### Suggested work

- Run the new `TestCancelTask` cases against a **synchronous** SUT and confirm the completed-task race no longer produces a false failure.
- Also check the restructured equivalent of `test_a2a_v030_new_methods.py::TestMethodMappingCompliance::test_core_method_mapping_compliance` (named in #140) for the same immediate-cancel race.
- If confirmed resolved, close #140 and the superseded draft #180. If it still reproduces, fix in `test_task_lifecycle.py` (not the deleted path).

_Follow-up from review of #180._