Harden POST-route test to assert CQL2 envelope shape

Replace `assert req.body is not None` (which passes for empty bytes,
malformed JSON, or any truthy value) with structural checks on the
parsed CQL2 body: top-level AND, the single IN predicate, the value
list in order. Also asserts the Content-Type header. Future
regressions in `_cql2_param` or routing fail loudly with a clear
message instead of slipping through.

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

Author: thodson-usgs

tests/waterdata_test.py

@@ -1,4 +1,5 @@
 import datetime
+import json
 import sys
 from unittest import mock
 
@@ -131,7 +132,18 @@ def test_construct_api_requests_monitoring_locations_post():
         hydrologic_unit_code=["010802050102", "010802050103"],
     )
     assert req.method == "POST"
-    assert req.body is not None
+    assert req.headers["Content-Type"] == "application/query-cql-json"
+
+    body = json.loads(req.body)
+    # Top-level shape: AND over a list of per-param predicates.
+    assert body["op"] == "and"
+    assert isinstance(body["args"], list) and len(body["args"]) == 1
+
+    # The single predicate is an IN over hydrologic_unit_code with both values.
+    predicate = body["args"][0]
+    assert predicate["op"] == "in"
+    assert predicate["args"][0] == {"property": "hydrologic_unit_code"}
+    assert predicate["args"][1] == ["010802050102", "010802050103"]
 
 
 def test_construct_api_requests_single_value_stays_get():