tests: serialise keyring writes across xdist workers

[robmsmt]

Problem

Main's post-merge run 33120371812 (rerun) died in 23 s with 12 passed, 90 errors, every one:

FileExistsError: [Errno 17] File exists: '/home/runner/.local/share/python_keyring'
  keyrings/alt/file_base.py:161 in _ensure_file_path -> os.makedirs(storage_root)
  <- keyring.set_password  <- InitConfig.set_value  <- conftest sml_config_dir (session, autouse)

sml_config_dir runs on each of the -n 2 xdist workers and writes secrets into the keyring; on CI that's keyrings.alt's plaintext file backend, whose _ensure_file_path calls os.makedirs without exist_ok. On a fresh runner both workers race for the directory; the loser raises, the session fixture error is cached, and every test on that worker errors. The backend's read-modify-write of keyring_pass.cfg isn't atomic either.

Fix

_keyring_write_lock() in tests/integration/conftest.py: if the active keyring exposes a file_path, create its directory with exist_ok=True and hold an fcntl.flock for the duration of the config writes. Backends without a file path (macOS Keychain locally) are untouched.

Verified locally with PYTHON_KEYRING_BACKEND=keyrings.alt.file.PlaintextKeyring and four concurrent processes on a fresh XDG_DATA_HOME: all writes land, file stays 0600.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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