Speech calls keep one connection per server, opened when listening starts

transcribe and speak share an httpx client per speech server with a 300 s
keep-alive, and run on the engine's loop. stt warms it with GET /v1/models
when an utterance opens, so a reply's first sentence finds the connection
open: a new one over a lossy link cost up to a second (openpanel V4).

backend/tests/flow/test_speech.py

@@ -52,6 +52,12 @@ def test_an_utterance_is_one_session_committed_when_listening_ends(monkeypatch):
         yield True, " Turn on the light."
 
     monkeypatch.setattr(client, "transcribe_stream", session)
+    warmed: list[str] = []
+
+    async def warm(api_base):
+        warmed.append(api_base)
+
+    monkeypatch.setattr(client, "warm", warm)
 
     async def scenario() -> None:
         node = SttNode(
@@ -94,6 +100,8 @@ def test_an_utterance_is_one_session_committed_when_listening_ends(monkeypatch):
         assert pipeline.state["voice.text"] == "Turn on the light."
         assert pipeline.state["voice.partial"] == "Turn on the light."
         assert node._session is None
+        # Opened with the session, for the reply's first sentence.
+        assert warmed == [f"{URL}/v1"]
 
         # No server, no session: the node says so instead.
         await hear(chunks[1], True, url="")
@@ -131,10 +139,16 @@ def test_a_sentence_is_spoken_once_and_the_rest_of_a_reply_queues(monkeypatch):
     Pipeline([node])
     node._artifacts = Store()
 
-    node.execute({"voice.reply": "Kitchen light on. Heating off.", "voice.url": URL})
-    # Long after both stopped playing.
-    node._until = -10.0
-    node.execute({"voice.reply": "Kitchen light on.", "voice.url": URL})
+    async def scenario() -> None:
+        node._loop = asyncio.get_running_loop()
+        reply = "Kitchen light on. Heating off."
+        await asyncio.to_thread(node.execute, {"voice.reply": reply, "voice.url": URL})
+        # Long after both stopped playing.
+        node._until = -10.0
+        reply = "Kitchen light on."
+        await asyncio.to_thread(node.execute, {"voice.reply": reply, "voice.url": URL})
+
+    asyncio.run(scenario())
 
     assert spoken == ["Kitchen light on.", "Heating off."]
     assert ["append" in speech for speech in published] == [False, True, False]