test: guard schema-probe allowlist against format drift

[millsmillsymills]

Adds tests/unit/test_schema_probe_allowlist.py, a guard test for the implicit coupling between compute_schema_drift (src/unraid_mcp/clients/unraid.py) and the fail-closed allowlist in the scrub step of .github/workflows/schema-probe.yml. Nothing in either file references the other, so a change to the drift message format desyncs the allowlist and the scrub then withholds every genuine drift report.

The test lifts four shell fragments out of the workflow verbatim — the field_list, allow_missing and allow_type assignments plus the unexpected= line carrying the grep -vE invocation — orders them by position in the file, and runs them under set -euo pipefail with the candidate lines on stdin. The patterns are therefore checked by a real shell rather than by a Python copy of them, and a change to the alternation or to the assignment order fails the test rather than being transcribed away. The CI run on ubuntu-latest is the authoritative one; a local run uses whatever grep is on PATH.

Coverage:

• Every drift line the shipped SCHEMA_EXPECTATIONS can produce passes the scrub. Three probe results span the output space: no type present, every type present but empty, and every type missing its alphabetically first field.
• The corpus contains both drift formats, so an all-pass result cannot be vacuous.
• A type with an empty expected field set (the [] list repr, reachable only through the type-missing branch) passes.
• Each drift format has its own set of rejected lookalikes. allow_missing rejects every missing fields lookalike on its own, so a corpus of those alone would leave allow_type with acceptance coverage only.
• A single unexpected line mixed in with valid drift is reported.
• _scrub_rejects asserts the shell wrote nothing to stderr, because a malformed pattern makes grep exit 2 with empty stdout, which the workflow's || true swallows.

No source or workflow changes.

Closes #293

🤖 Generated with Claude Code