fix: RaisesGroup check() no longer called with sub-exceptions (#14324)

[DawnofGenX]

Summary

Fixes #14324.

RaisesGroup's check callback was being called a second time with a contained sub-exception after the callback had already rejected the group. This came from a speculative probe used to build a nicer failure hint ("did return True for the expected ... You might want RaisesGroup(RaisesExc(..., check=<...>))"). It:

• violated the documented contract that check receives the exception group itself,
• crashed callbacks that assume they are always handed a group (e.g. touching e.exceptions raised AttributeError on the sub-exception and leaked out of pytest.raises matching), and
• caused surprising double-calls for stateful callbacks.

Fix

The speculative sub-exception probe is removed entirely — not merely wrapped in try/except. When check rejects the group, the failure reason is now set directly and check is never invoked again. This drops the "You might want RaisesGroup(RaisesExc(...))" hint, but restores the contract and eliminates the whole crash class.

Changes

• src/_pytest/raises.py — remove the sub-exception probe in RaisesGroup check handling.
• testing/python/raises_group.py — regression test asserting check is only ever called with the group and never crashes when it uses group-only attributes.
• changelog/14324.bugfix.rst — changelog entry.

Test plan

• New regression test test_check_not_called_with_subexception_issue_14324 fails on main (verified red: AttributeError: 'ValueError' object has no attribute 'exceptions') and passes with the fix.
• testing/python/raises_group.py + testing/python/raises.py pass in full (67 passed).
• CI green.

Closes #14324