test(conftest): #160 reset module-level caches between tests (xdist hardening)

CI failures on PR #168 after rebasing onto main + PR #169/#170:
gw2 worker bucket reproducibly fails test_admin_can_list_registry +
test_three_sources_catalog_count with `assert "X" in set()` — the
register-table POST landed but list/catalog endpoints returned empty.

Root cause: pre-existing module-level cache leak across tests on the
same xdist worker process. `app.instance_config._instance_config`,
`connectors.bigquery.access.get_bq_access` (functools.cache), and
`app.api.v2_quota._quota_singleton` all survive across function-scoped
fixtures, so a prior test that read instance.yaml against an old
DATA_DIR poisons the next test's env even after `monkeypatch.setenv`
resets DATA_DIR.

Pre-existing on main — surfaced now because #160's new tests changed
the xdist test bucket distribution and dropped a different mix of
tests onto gw2 that hit the leak. Direct cause is unchanged; my T1a
fix in test_main_exits_when_project_missing addressed one symptom of
the same pollution but didn't generalize.

Add an autouse fixture in conftest.py that resets all three caches
before every test. Generic fix; helps any future test that reads
instance.yaml or BqAccess and would otherwise be order-dependent on
the worker.

tests/conftest.py

@@ -47,6 +47,42 @@ def _disable_auth_rate_limit_in_tests():
     limiter.enabled = was_enabled
 
 
+@pytest.fixture(autouse=True)
+def _reset_module_caches():
+    """Reset module-level caches that survive across tests on the same
+    pytest-xdist worker process. Without this, a test that populates
+    `app.instance_config._instance_config` (e.g. via `runpy.run_module`
+    in test_bigquery_extractor's __main__ tests, or via any path that
+    calls `app.instance_config.get_value`) leaves stale config visible
+    to the next test on that worker — including config that points at
+    a different DATA_DIR than the next test's e2e_env set.
+
+    Caches reset:
+    - app.instance_config._instance_config — instance.yaml deep-merge cache
+    - get_bq_access (functools.cache) — BqAccess(BqProjects(...)) lru
+    - app.api.v2_quota._quota_singleton — per-user quota tracker
+
+    Pre-existing flakiness; surfaced by issue #160 PR #168 shifting the
+    test bucket distribution on xdist worker gw2.
+    """
+    try:
+        import app.instance_config as _ic
+        _ic._instance_config = None
+        try:
+            from connectors.bigquery.access import get_bq_access
+            get_bq_access.cache_clear()
+        except (ImportError, AttributeError):
+            pass
+    except ImportError:
+        pass
+    try:
+        import app.api.v2_quota as _q
+        _q._quota_singleton = None
+    except ImportError:
+        pass
+    yield
+
+
 @pytest.fixture
 def e2e_env(tmp_path, monkeypatch):
     """Set up complete E2E environment with DATA_DIR, create dirs."""