Fix fixture finalizers skipped on KeyboardInterrupt during setup (#15067)

[Abelastro]

What

Fix two defects that caused request.addfinalizer() finalizers to be silently skipped when fixture setup is interrupted by a BaseException such as KeyboardInterrupt:

1. src/_pytest/runner.py — runtestprotocol() now runs the item teardown phase in a finally block, so teardown (and therefore fixture finalizers) executes even when setup or call re-raises an interruptible exception. Previously the KeyboardInterrupt propagated out of the setup phase and teardown never ran.
2. src/_pytest/fixtures.py — FixtureDef.finish() only short-circuits when the fixture has no cached result and no registered finalizers. A setup interrupted by a BaseException has no cached result but may still hold pending finalizers; those must run and be cleared instead of being skipped and leaked.

Why

Per the documented behavior of request.addfinalizer(), a finalizer registered during setup must run at teardown no matter how setup completes. Repro:

@pytest.fixture
def resource(request):
    request.addfinalizer(lambda: Path("marker.txt").write_text("ran"))
    raise KeyboardInterrupt

Running this test previously aborted the session with !!!!!!!! KeyboardInterrupt !!!!!!!!!!! and left marker.txt missing. The finalizers are now executed ([#15067]).

Changes

• src/_pytest/runner.py: teardown call moved into a finally (keeping the existing #11706 behavior for session fail/stop), reports initialized before the try.
• src/_pytest/fixtures.py: finish() guard now if self.cached_result is None and not self._finalizers.
• testing/python/fixtures.py: regression test test_finalizer_runs_when_setup_interrupted_by_keyboard_interrupt asserting the finalizer marker is written after runtestprotocol re-raises KeyboardInterrupt.
• changelog/15067.bugfix.rst.

Verified against pytest's own suite: testing/test_runner.py + testing/python/fixtures.py (312 tests) pass with no new failures.