fix(api-server): share one Docker container across all API conversations (#7127)

The API server's _run_agent() was not passing task_id to
run_conversation(), causing a fresh random UUID per request. This meant
every Open WebUI message spun up a new Docker container and tore it down
afterward — making persistent filesystem state impossible.

Two fixes:

1. Pass task_id="default" so all API server conversations share the same
   Docker container (matching the design intent: one configured Docker
   environment, always the same container).

2. Derive a stable session_id from the system prompt + first user message
   hash instead of uuid4(). This stops hermes sessions list from being
   polluted with single-message throwaway sessions.

Fixes #3438.

tests/gateway/test_api_server.py

@@ -26,6 +26,7 @@ from gateway.platforms.api_server import (
     APIServerAdapter,
     ResponseStore,
     _CORS_HEADERS,
+    _derive_chat_session_id,
     check_api_server_requirements,
     cors_middleware,
     security_headers_middleware,
@@ -658,6 +659,98 @@ class TestChatCompletionsEndpoint:
             data = await resp.json()
             assert "Provider failed" in data["error"]["message"]
 
+    @pytest.mark.asyncio
+    async def test_stable_session_id_across_turns(self, adapter):
+        """Same conversation (same first user message) produces the same session_id."""
+        mock_result = {"final_response": "ok", "messages": [], "api_calls": 1}
+
+        app = _create_app(adapter)
+        session_ids = []
+        async with TestClient(TestServer(app)) as cli:
+            # Turn 1: single user message
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+                await cli.post(
+                    "/v1/chat/completions",
+                    json={
+                        "model": "hermes-agent",
+                        "messages": [{"role": "user", "content": "Hello"}],
+                    },
+                )
+                session_ids.append(mock_run.call_args.kwargs["session_id"])
+
+            # Turn 2: same first message, conversation grew
+            with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+                await cli.post(
+                    "/v1/chat/completions",
+                    json={
+                        "model": "hermes-agent",
+                        "messages": [
+                            {"role": "user", "content": "Hello"},
+                            {"role": "assistant", "content": "Hi there!"},
+                            {"role": "user", "content": "How are you?"},
+                        ],
+                    },
+                )
+                session_ids.append(mock_run.call_args.kwargs["session_id"])
+
+        assert session_ids[0] == session_ids[1], "Session ID should be stable across turns"
+        assert session_ids[0].startswith("api-"), "Derived session IDs should have api- prefix"
+
+    @pytest.mark.asyncio
+    async def test_different_conversations_get_different_session_ids(self, adapter):
+        """Different first messages produce different session_ids."""
+        mock_result = {"final_response": "ok", "messages": [], "api_calls": 1}
+
+        app = _create_app(adapter)
+        session_ids = []
+        async with TestClient(TestServer(app)) as cli:
+            for first_msg in ["Hello", "Goodbye"]:
+                with patch.object(adapter, "_run_agent", new_callable=AsyncMock) as mock_run:
+                    mock_run.return_value = (mock_result, {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0})
+                    await cli.post(
+                        "/v1/chat/completions",
+                        json={
+                            "model": "hermes-agent",
+                            "messages": [{"role": "user", "content": first_msg}],
+                        },
+                    )
+                    session_ids.append(mock_run.call_args.kwargs["session_id"])
+
+        assert session_ids[0] != session_ids[1]
+
+
+# ---------------------------------------------------------------------------
+# _derive_chat_session_id unit tests
+# ---------------------------------------------------------------------------
+
+
+class TestDeriveChatSessionId:
+    def test_deterministic(self):
+        """Same inputs always produce the same session ID."""
+        a = _derive_chat_session_id("sys", "hello")
+        b = _derive_chat_session_id("sys", "hello")
+        assert a == b
+
+    def test_prefix(self):
+        assert _derive_chat_session_id(None, "hi").startswith("api-")
+
+    def test_different_system_prompt(self):
+        a = _derive_chat_session_id("You are a pirate.", "Hello")
+        b = _derive_chat_session_id("You are a robot.", "Hello")
+        assert a != b
+
+    def test_different_first_message(self):
+        a = _derive_chat_session_id(None, "Hello")
+        b = _derive_chat_session_id(None, "Goodbye")
+        assert a != b
+
+    def test_none_system_prompt(self):
+        """None system prompt doesn't crash."""
+        sid = _derive_chat_session_id(None, "test")
+        assert isinstance(sid, str) and len(sid) > 4
+
 
 # ---------------------------------------------------------------------------
 # /v1/responses endpoint