Replace conftest auto-marking with module-level pytestmark

The previous conftest hook auto-applied pytest.mark.flaky to tests
by inspecting function source for references to a central
_PUBLIC_GETTERS frozenset. This was implicit and required keeping
that set in sync with the public API. Switch to explicit opt-in: drop
conftest.py and declare the marker at the top of waterdata_test.py
via pytestmark. The other waterdata_*_test.py files are entirely
mocked or pure unit tests and need no retry.

The retry config (reruns count, delay, transient-error trace
patterns) is co-located with the tests it protects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

tests/conftest.py

@@ -33,6 +33,24 @@
     _normalize_str_iterable,
 )
 
+# Most tests in this module call the live USGS Water Data API. After
+# PR #273, transient upstream errors (5xx / 429 / connection drops)
+# propagate instead of silently truncating, which makes CI susceptible
+# to flaking on a brief upstream blip. Auto-retry such failures, but
+# only for the narrow set of transient-error trace patterns — library
+# bugs raising other exception types still fail on the first try.
+# Tests in this file that use ``requests_mock`` or ``mock.patch`` won't
+# match these patterns and so are effectively unaffected.
+pytestmark = pytest.mark.flaky(
+    reruns=2,
+    reruns_delay=5,
+    only_rerun=[
+        r"RuntimeError:\s*(?:429|5\d\d):",  # _raise_for_non_200 output
+        r"ConnectionError",
+        r"ReadTimeout|ConnectTimeout|Timeout",
+    ],
+)
+
 
 def mock_request(requests_mock, request_url, file_path):
     """Mock request code"""