fix(test): drop the one genuinely-unused _seed_session_dir binding (lint)

CI lint (diff-scoped ruff F) flagged 'models =' unused at line 92 (that test
uses only the seed side-effect). Removed the assignment there; kept it at line
320 where test_clear_survives_startup_recovery legitimately reads models.SESSION_DIR.
Ruff clean, 11 #5532 tests pass.

tests/test_issue5532_clear_truncation_watermark.py

@@ -89,7 +89,7 @@ def test_clear_endpoint_sets_truncation_watermark(monkeypatch, tmp_path):
     This is the fail-without-fix assertion: on origin/master the watermark
     stays None because the handler never set it.
     """
-    models = _seed_session_dir(monkeypatch, tmp_path)
+    _seed_session_dir(monkeypatch, tmp_path)
     from api.models import Session
 
     session = Session(