ci(unit-tests): drop stale tests/test_wl_hmac_key.py reference

The "Run module-level tests" step in ci.yml referenced
tests/test_wl_hmac_key.py — a file that never existed. The result
was every Unit Tests run on main red since the workflow landed
(verified via gh run list --workflow=ci.yml -- 5+ consecutive
failures on commits a48f6c2 / 4330149 / ab53066 / 91d9d22 /
b14b3c9 all citing "ERROR: file or directory not found:
tests/test_wl_hmac_key.py", exit code 4).

HMAC coverage is already provided by tests/unit/test_hmac_key.py
+ tests/unit/test_hmac_sig_fuzz.py, both of which run via the
prior `pytest tests/unit/` step. So removing the broken line
loses zero coverage — it just unblocks the workflow.

Same shape of fix as the round-7-B4 cleanup that removed a
stale tests/test_wl_filelock.py reference. Updated the
preamble comment to document BOTH missing-file gotchas so
they don't get re-added by a future contributor reading the
old comment as a TODO.

Side benefit: this also unblocks the Unit Tests check on every
open Dependabot PR (#7-#16), which were all red on the same
missing-file error.

Author: RelativisticJet

.github/workflows/ci.yml

@@ -129,15 +129,24 @@ jobs:
 
       - name: Run module-level tests
         # Top-level tests/test_*.py files cover lower-layer modules
-        # (wl_limits, wl_hmac_key, wl_fim_common, wl_expiration_cleanup)
-        # with HMAC and period-boundary coverage that tests/unit/ does
-        # not duplicate. Note: a former tests/test_wl_filelock.py
-        # reference was removed during round 7 B4 — no such file
-        # exists; the filelock paths are exercised inside
-        # tests/unit/test_filelock.py.
+        # (wl_limits, wl_fim_common, wl_expiration_cleanup) with
+        # period-boundary + scheduled-input coverage that tests/unit/
+        # does not duplicate.
+        #
+        # Notes on tests that LOOK like they should be in this list
+        # but aren't:
+        #   - tests/test_wl_filelock.py: no such file ever existed
+        #     (filelock paths are exercised inside
+        #     tests/unit/test_filelock.py which runs in the prior step).
+        #   - tests/test_wl_hmac_key.py: same — no such file. HMAC
+        #     coverage lives at tests/unit/test_hmac_key.py +
+        #     tests/unit/test_hmac_sig_fuzz.py, both of which run in
+        #     the prior `pytest tests/unit/` step. A stale reference
+        #     to the non-existent top-level path was removed
+        #     2026-05-22 — it was the cause of every red Unit Tests
+        #     run on main since the workflow landed.
         run: |
           python -m pytest tests/test_wl_limits.py \
-                          tests/test_wl_hmac_key.py \
                           tests/test_wl_fim_common.py \
                           tests/test_wl_expiration_cleanup.py \
                           -q --tb=short