fix: deduplicate Directory nodes on re-collection to preserve fixture identity (#14635)

[RonnyPfannschmidt]

Summary

Fixes #14635.

When Session re-collects a parent Directory (due to handle_dupes=False for file CLI args), fresh child nodes were created. Since fixture registration uses node identity for matching (_matchfactories checks fixturedef.node in parent_nodes), the new Directory children didn't match fixtures registered with the original instances.

This caused fixture closure computation to fail for tests collected after unrelated paths, because the conftest fixtures were registered with a now-orphaned Directory node instance.

Root cause: _collect_one_node with handle_dupes=False calls collect() on the parent Directory, which creates brand-new child Directory/Package instances. The _collection_cache is then updated with these new children. Later collection paths that look up cached children find different node objects than the ones conftest fixtures were registered with.

Fix: After re-collection, replace any freshly-created Directory children with the previously-seen instances for the same path. Module/File nodes are still recreated to preserve --keep-duplicates semantics (which needs fresh test items).

Test plan

• Added regression test test_fixture_closure_order_independence_with_parametrize that reproduces the exact scenario from Error collecting tests on 9.1.1 - fixtures not found when a parent directory appears multiple times in a argument list  #14635
• Existing TestOverlappingCollectionArguments::test_complex_combined_handling (which exercises --keep-duplicates with overlapping file/dir args) passes
• --keep-duplicates file.py file.py still collects the file twice
• All fixture and conftest tests pass (274 tests)
• All collection tests pass

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