Fix monkeypatch recording failed mutations

[dhootraghav]

Summary

Fix MonkeyPatch methods recording undo state before the underlying mutation succeeds.

Previously, delattr, setitem, and delitem recorded their undo information before performing the mutation. If the mutation raised an exception, the failed operation was still recorded and later replayed by undo(), which could raise a second unexpected exception.

This change records the undo state only after the mutation succeeds.

Changes

• Record delattr undo state after successful deletion.
• Record setitem undo state after successful assignment.
• Record delitem undo state after successful deletion.
• Add regression tests covering failed mutations for all three operations.

Testing

python -m pytest testing/test_monkeypatch.py -q
37 passed, 2 skipped

also verified git diff --check with no errors

### 4. Before clicking **Create pull request**

Look at the **Files changed** tab.

You should see only:

```text
src/_pytest/monkeypatch.py
testing/test_monkeypatch.py