test(api-server): keep the behavioural run-output test, drop the source-reading guard

The salvaged commit shipped two tests: a disconnect-mid-stream test that reads
the reply back through GET /v1/runs/{run_id}, and a structural guard that
opened api_server.py and searched its text for `output=`. Tests that read
source are change-detectors, not invariants; the behavioural test already
fails the moment `output` disappears from the terminal write, so the guard is
gone. The inline comment shrinks to the WHY (parity with /v1/runs, the
recovery path) and drops the narrative.

tests/gateway/test_session_api.py

@@ -1092,42 +1092,3 @@ async def test_session_stream_records_reply_text_for_post_disconnect_recovery(
     response = await adapter._handle_get_run(get_request)
     assert response.status == 200
     assert "the answer worth keeping" in response.text
-
-
-@pytest.mark.asyncio
-async def test_both_run_routes_record_output(adapter):
-    """Structural guard against the asymmetry coming back.
-
-    The two routes that mint run ids must both persist the reply text. A future
-    edit that drops `output=` from either one silently removes the recovery
-    path, and no behavioural test of the *other* route would notice.
-    """
-    from pathlib import Path
-
-    src = (
-        Path(__file__).resolve().parents[2]
-        / "gateway"
-        / "platforms"
-        / "api_server.py"
-    ).read_text(encoding="utf-8")
-
-    for name, anchor in (
-        ("session stream", "async def _handle_session_chat_stream"),
-        ("/v1/runs", "async def _handle_runs"),
-    ):
-        start = src.index(anchor)
-        # Anchor on the terminal STATUS WRITE, not on the string "completed" —
-        # the SSE payload carries a `"completed": True` key that would match a
-        # naive search and hide a missing output=.
-        cursor = start
-        window = None
-        while True:
-            call_at = src.find("_set_run_status(", cursor)
-            assert call_at != -1, f"{name}: no terminal _set_run_status found"
-            candidate = src[call_at:call_at + 2000]  # generous: the call may carry comments
-            if '"completed",' in candidate[:400]:
-                window = candidate
-                break
-            cursor = call_at + 1
-
-        assert "output=" in window, f"{name} no longer records the reply text"