Fall back to AJAX on a Moodle course-context validation error

[erseco]

Summary

list_courses()'s webservice-to-AJAX fallback only triggered on an invalid/expired token. Broadens it to also cover a real, CI-observed Moodle course-context validation quirk, so callers transparently recover via AJAX instead of erroring out.

Linked issue

Closes #63

Specification

See #63 for the full write-up and CI evidence (two separate failed runs, three different tests affected, all on PR #62).

Implementation details

• src/py_moodle/transport/webservice.py: renamed the single _INVALID_TOKEN_MARKER to a _TRANSPORT_UNAVAILABLE_MARKERS tuple, adding "cannot execute functions in the course context" (the stable, generic part of the Moodle message — the course id and exact sub-message vary) alongside the existing invalid-token marker. Both now raise TransportUnavailableError (fall back to AJAX) instead of TransportError (raise immediately).
• tests/unit/test_transport.py: two new regression tests reproducing the exact error message seen in CI, at both the webservice_transport.call() level and the public list_courses() level.

Test plan

• Added the failing tests first.
• Confirmed they failed against the pre-fix code (via git stash on just the source file) — reproduced the exact CI traceback shape.
• Implemented the fix (broadened marker list).
• Confirmed the two new tests pass, plus all 10 tests in test_transport.py.
• Confirmed the full pytest tests/unit suite (199 tests) and make lint are still green.

Commands run:

pytest tests/unit/test_transport.py -k context_error -q   # red, via git stash on webservice.py
git stash pop
pytest tests/unit/test_transport.py -q                     # green, all 10 pass
pytest tests/unit -q                                        # 199 passed
black --check src tests && isort --check-only src tests && flake8

Backward compatibility

Fully backward compatible. Only broadens which errors trigger the existing AJAX fallback path (already exercised by test_list_courses_falls_back_to_ajax_on_invalid_token); does not change list_courses()'s signature, return shape, or any other function's behavior. Genuine, non-context, non-token TransportErrors (e.g. permissions denials) still raise immediately, unchanged, per the existing test_webservice_call_raises_transport_error_on_other_moodle_exception test.

Risk assessment

The marker-string approach matches this file's existing, already-reviewed convention (_INVALID_TOKEN_MARKER) rather than introducing a new classification mechanism. As with any English-substring match on a Moodle error message, it's not immune to locale/wording drift in a future Moodle release — same caveat that already applied to the invalid-token marker before this PR.

Manual verification

Can't reproduce this specific race against a live Moodle instance from this environment on demand (it's timing-dependent), but the CI evidence in #63 is the real-world reproduction. The regression test in test_transport.py reproduces the exact error message deterministically without needing live Moodle or a race.

Notes for reviewers

This was discovered while verifying PR #62 (test de-flaking), not something #62 introduced — #62 is unrelated test-fixture code; this is a production src/py_moodle/transport/webservice.py fix. Kept as its own focused PR rather than folded into #62.