TaskGroup: tg.cancel() shouldn't block create_task() allow cooperative cancellation

Currently, when a TaskGroup is explicitly cancelled via tg.cancel(), any subsequent call to tg.create_task() raises:

RuntimeError: TaskGroup <TaskGroup cancelling> is shutting down

This contradicts the TODO in the test test_taskgroup_cancel_before_create_task, which states:

This behavior is not ideal. We'd rather have no exception raised, and the child task run until the first await.

Proposed change:
After an explicit tg.cancel() (not after a failure-induced abort), create_task() should succeed. The new task should run its synchronous code up to the first await, then receive CancelledError

For failure-driven aborts (e.g., an exception in another task), the existing RuntimeError is preserved to prevent unsafe task creation during error cleanup.

A PR implementing this change (with a new _explicitly_cancelled flag) is ready and passes all existing tests.

This resolves the TODO and makes TaskGroup behavior more intuitive when voluntarily cancelled.

Linked PRs

• gh-150357

Bắt đầu với TODO test_taskgroup_cancel_before_create_task và các entry point cancel/create_task của TaskGroup được mô tả trong issue. So sánh tg.cancel() tường minh với hành vi abort do lỗi kích hoạt. Hoàn thành khi một task được tạo sau khi hủy tường minh chạy qua mã đồng bộ của nó trước khi nhận CancelledError, còn các lần abort do lỗi kích hoạt vẫn raise RuntimeError.