tests: cover login and credential-storage edge branches

[Faycall1l]

Summary

Adds unit tests for the two lowest-coverage files in the codebase, paperpush/venues/login.py (+32 tests, 52% → 94%) and paperpush/credentials.py (69% → 98%), using the existing fake-playwright test infrastructure (_FakePage/_FakeLocator). No production code changes; a venv-free, hermetic test-only PR.

What's covered

verify_login driver (tests/test_login.py)

• Successful sign-in saves the session and closes the browser.
• headless= is passed through to the venue's login.
• The ORCID branch (method="orcid") passes orcid=True down; refused when the venue lacks supports_orcid_login (same orcid_unsupported message the submission path produces).
• No automated sign-in available → LoginVerificationError naming the venue.
• Headless failure → LoginVerificationError with the underlying cause; headed failure → prompt for a human, re-check, then save only if logged in.
• Sign-in that does not take → LoginVerificationError("...did not take...").
• NotImplementedError from the venue propagates unchanged.

paperpush/credentials.py (keyring + file store)

• save_credential: writes to keyring with service name paperpush:<slug>; falls back to the file store when the keyring rejects a write or a read.
• get_credential/delete_credential: clear keyring + file store; incomplete keyring entry / corrupt file store treated as absent/empty.
• _get_keyring branch coverage: keyring module missing (ImportError), backend lookup failure, failing backend, working backend.

paperpush/venues/login.py helpers

• first_present / first_visible fast/slow paths.
• fill_login_form (shared by the login page): success and missing-control VenueLoginError.
• login_orcid: popup control absent and ORCID form field missing → clean popup close.

Validation

• pytest: 1382 passed / 74 skipped (was 1350); all portal tests still skipped by default; zero network or OS-keyring access (fixtures are additive only).
• black . -l 99999 --check: clean.
• All four generated-content checks remain green (gen_readme_venues.py, gen_venues_schema.py, gen_schema_docs.py, gen_subfile_templates.py) — no generated files touched.
• Verified on Python 3.10 locally.