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

Comienza con el TODO test_taskgroup_cancel_before_create_task y los puntos de entrada de TaskGroup cancel/create_task descritos en el issue. Compara tg.cancel() explícito con el comportamiento de abortar por fallo. Se considera terminado cuando una tarea creada después de la cancelación explícita ejecuta su código síncrono antes de recibir CancelledError, mientras que los abortos provocados por fallos siguen generando RuntimeError.