Load initial conftest when an option value is an existing file path

[breidenbach0]

Problem

Fixes #13913.

Since pytest 9 (commit 115b8703, switch to intermixed argument parsing), a conftest.py that registers command line options is sometimes not loaded as an initial conftest, so its options are reported as unrecognized arguments. Reported by SQLAlchemy's lead (@zzzeek) and confirmed by at least one other project.

Reproducer (minimal):

tests/conftest.py   # defines --write-idents via pytest_addoption
test_it.py
idents.txt          # an existing file

$ pytest --write-idents idents.txt
ERROR: ... unrecognized arguments: --write-idents

Reproduced on current main.

Cause

(as diagnosed by @bluetech in the issue)

With intermixed parsing, the value of an option that is a path to an existing file ends up among the positional args during pre-parsing:

file_or_dir = ['idents.txt']      # was in `unknown` before intermixed parsing
unknown     = ['--write-idents']

_set_initial_conftests() then treats idents.txt as a test-path anchor. Because it is a file:

1. anchor.is_dir() is false → the test* sub-directory glob is skipped, and
2. the anchor list is non-empty → the invocation-directory test* fallback is skipped too.

So the test*/conftest.py that registers --write-idents is never discovered.

Fix

Resolve a file anchor to its containing directory (_get_directory) so both the conftest loading and the test* sub-directory discovery consider the right location. This follows the fix direction @bluetech sketched in the issue.

The only behavioral change is that a file argument also triggers test* discovery relative to its directory — consistent with the existing test* auto-discovery feature (#14608) and the documented behavior; the full test suite is unaffected.

Tests

• New regression test test_initial_conftest_loaded_when_option_value_is_existing_file (passes with the fix, fails without).
• The pre-existing test_conftests_in_invocation_dir_tests_is_initial (pytest 9.1.0: conftest.py addoption regressed in nested directories #14608) still passes.
• testing/test_conftest.py, test_collection.py, test_main.py, test_config.py, and a broad sweep (test_capture, test_terminal, test_session, test_runner, test_debugging, test_assertion, python/collect, python/fixtures) show no new failures versus baseline (identical pass/error counts; the only errors/failures are pre-existing and unrelated — the dev minversion/pytest.toml-parse environment issues).

News fragment changelog/13913.bugfix.rst added.

cc @bluetech @zzzeek