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

從 test_taskgroup_cancel_before_create_task TODO 以及 issue 中描述的 TaskGroup cancel/create_task 入口開始。比較明確的 tg.cancel() 與由失敗驅動的中止行為。完成標準是：明確取消後建立的工作在接收 CancelledError 之前會執行其同步程式碼，而由失敗驅動的中止仍會引發 RuntimeError。