fix: preserve idempotent admission under capacity and preparation failures

tests/unit/test_chat_api.py

@@ -90,6 +90,13 @@ def test_idempotent_send_final_output_and_redacted_replay(env):
     assert 'event: done' in stream.text
     assert 'hidden-secret' not in stream.text and 'tool-secret' not in stream.text
     assert 'secret-never-public' not in stream.text
+    # Capacity limits never prevent retrieval of an already accepted operation.
+    async def fill():
+        app.state.chat_runs.tasks.update({str(i): asyncio.create_task(asyncio.sleep(100)) for i in range(256)})
+    client.portal.call(fill)
+    replay = client.post(url, headers=headers, json=value)
+    assert replay.status_code == 202
+    assert replay.json()['run']['id'] == rid
 
 def test_body_validation_and_honest_capabilities(env):
     client, app, adapter, headers = env