Runs export: a truncated one counts the selection into X-Total-Count; the SDK and fluksio export warn how many runs were left out

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/tests/api/routes/test_runs.py

@@ -1477,3 +1477,26 @@ def test_runs_page_by_when_they_were_created(client, superuser_token_headers, pa
         "page-2",
         "page-1",
     ]
+
+
+@pytest.mark.usefixtures("paged")
+def test_a_truncated_export_says_how_many_runs_it_left_out(
+    client, superuser_token_headers, monkeypatch
+):
+    """The cap is in one header, the selection's size in another, and the SDK
+    turns the two into a warning naming what is missing."""
+    from fluksio.sdk.client import Client
+
+    monkeypatch.setattr("fluksio.api.routes.runs.EXPORT_CAP", 2)
+    answer = client.get(
+        f"{settings.API_V1_STR}/runs/export/runs",
+        params={"flow": "paged-study", "format": "jsonl"},
+        headers=superuser_token_headers,
+    )
+    assert answer.headers["X-Truncated"] == "2"
+    assert answer.headers["X-Total-Count"] == "3"
+    assert [row["id"] for row in _lines(answer)] == ["page-2", "page-1"]
+
+    sdk = Client(http=client, token=superuser_token_headers["Authorization"][7:])
+    with pytest.warns(UserWarning, match="1 older runs"):
+        assert len(sdk.export_runs(flow="paged-study")) == 2