Avoid rechecking RaisesGroup callbacks

[raashish1601]

Fixes #14324.

Summary:

• Keep RaisesGroup.matches() from re-calling the user check callback on actual_exceptions[0] just to build a suggestion.
• Preserve the group-level check evaluation and derive the RaisesExc(...) hint without invoking user code again.

Validation:

• python -m pytest testing/python/raises_group.py::test_check testing/python/raises_group.py::test_check_is_only_called_on_the_group testing/python/raises_group.py::test_unwrapped_match_check -q
• python -m pytest testing/python/raises_group.py::test_raises_group -q

Manual risk:

• Behavior is covered by targeted regression tests, but the usual upstream CI review is still the final gate.