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

Inizia con il TODO test_taskgroup_cancel_before_create_task e con gli entry point cancel/create_task di TaskGroup descritti nell’issue. Confronta tg.cancel() esplicito con il comportamento di abort determinato da un errore. Il lavoro è completato quando un task creato dopo la cancellazione esplicita esegue il proprio codice sincrono prima di ricevere CancelledError, mentre gli abort determinati da errori continuano a sollevare RuntimeError.