Fix nested caplog.filtering with same filter object

[elpolini]

Summary

• Fix caplog.filtering so nested use with the same filter object does not remove the outer filter too early.
• Only add/remove the filter when this context manager actually changed handler state.
• Add a regression test for nested caplog.filtering with the same filter instance.

Why

Issue #14189 reports that nested caplog.filtering can unexpectedly remove an active filter, causing logs to leak into capture inside the outer context.

Validation

• python -m pytest -q testing/logging/test_fixture.py -k with_statement_filtering