Fix quadratic collection when passing many file arguments from one directory

[suryakant-source]

closes #15187

Problem

When several test files from the same directory are passed as explicit command-line arguments (e.g. pytest a/test_1.py a/test_2.py ...), Session.collect() re-collects the parent directory once per file argument. Cost grows with (number of file arguments) x (number of entries in the directory), so a CI job passing ~100 explicit files from a ~940-entry directory spends its collection time re-scanning the same directory ~100 times. Reported in #15187 and again in #15221.

Root cause

In Session.collect(), the parent of the last path part of a file argument is collected with handle_dupes=False so that files given directly multiple times on the command line are not deduplicated. That forces a full re-collection of the parent directory for every file argument, even though only the one requested file is needed.

Fix

In the default mode, normalize_collection_arguments() has already removed duplicate and overlapping arguments, so the cached collection of the parent directory can always be reused. The fresh re-collection is only required with --keep-duplicates, so handle_dupes=False is now gated on that option.

Measured with 200 test files in one directory and a counting pytest_ignore_collect hook:

file args	directory-entry scans before	scans after
10	1902	193
20	3622	183
40	6442	163

Verification

• New regression test test_multiple_file_args_do_not_rescan_parent_directory fails without the fix (~400 pytest_collect_file hook calls for 20 files) and passes with it.
• Duplicate semantics unchanged, re-verified after the patch: --keep-duplicates with overlapping file+dir args still runs the file twice; pytest a.py a.py still runs the file once; pytest a.py::test_x a.py::test_y still selects exactly the requested items.
• testing/test_collection.py, test_session.py, test_pathlib.py, test_conftest.py, test_config.py all pass locally (621 passed, plus the new test).

Checklist:

• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits.
• Added closes #15187 above.
• AI agents were used; they are credited in Co-authored-by commit trailers, per the AI/LLM-Assisted Contributions Policy.
• New changelog file changelog/15187.bugfix.rst.
• Added myself to AUTHORS.