fix(tests): use exact-equality check to clear CodeQL substring-URL false positive (#1183)

CodeQL (py/incomplete-url-substring-sanitization) flagged the "https://schema.org/"
in flattened check because it pattern-matches on URL-ish strings tested with `in`.
flattened is always a list here, so the check was already exact membership, not a
substring test on untrusted input, but the ambiguous idiom tripped the scanner.
Rewrite as an explicit equality comparison so the intent is unambiguous.

tests/export/test_jsonld_default_graph.py

@@ -160,7 +160,7 @@ def test_a_url_valued_context_is_not_thrown_away(tmp_path):
     context = json.loads(path.read_text())["@context"]
 
     flattened = context if isinstance(context, list) else [context]
-    assert "https://schema.org/" in flattened, (
+    assert any(entry == "https://schema.org/" for entry in flattened), (
         "the caller's context was replaced by Semantica's defaults, "
         "which silently changes how every term expands"
     )