feat: add temporal context and contextual help

tests/test_conditional_requests.py

@@ -48,6 +48,10 @@ class ConditionalRequestTests(unittest.TestCase):
             self.assertIn("private", first.headers.get("cache-control", ""))
             self.assertIn("no-cache", first.headers.get("cache-control", ""))
             self.assertIn("authorization", first.headers.get("vary", "").lower())
+            self.assertIn(
+                "x-govoplan-validity-mode",
+                first.headers.get("vary", "").lower(),
+            )
             self.assertEqual("request-1", first.headers["X-Correlation-ID"])
 
             second = client.get("/json", headers={"If-None-Match": etag or "", "X-Request-ID": "request-2"})
@@ -56,6 +60,20 @@ class ConditionalRequestTests(unittest.TestCase):
             self.assertEqual(etag, second.headers.get("etag"))
             self.assertEqual("request-2", second.headers["X-Correlation-ID"])
 
+            historical = client.get(
+                "/json",
+                headers={
+                    "X-Govoplan-Validity-Mode": "at",
+                    "X-Govoplan-Valid-At": "2025-02-03T10:30:00Z",
+                },
+            )
+            self.assertEqual(200, historical.status_code, historical.text)
+            self.assertEqual("at", historical.headers["X-Govoplan-Validity-Mode"])
+            self.assertIn(
+                "x-govoplan-valid-at",
+                historical.headers.get("vary", "").lower(),
+            )
+
     def test_changed_json_body_does_not_match_previous_etag(self) -> None:
         with self._client() as client:
             first = client.get("/json?value=alpha")