feat: add OpenAI-compatible gateway

- GET /v1/health, GET /v1/models, POST /v1/chat/completions (streaming + non-streaming)
- OpenAI SDK, LangChain ChatOpenAI, and SSE streaming integration tests
- Tracker-backed GET /v1/models endpoint
- OpenAI-format errors for unavailable model (503) and pipeline failures
- Malformed JSON body handled with 400 instead of crash
- Test deps (openai, langchain-openai) declared in root pyproject dev extras

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/tracker/meshnet_tracker/server.py

@@ -151,10 +151,21 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
             self._handle_route(parsed)
         elif parsed.path == "/v1/nodes/assign":
             self._handle_assign(parsed)
+        elif parsed.path == "/v1/models":
+            self._handle_models()
         else:
             self.send_response(404)
             self.end_headers()
 
+    def _handle_models(self):
+        server: _TrackerHTTPServer = self.server  # type: ignore[assignment]
+        created = int(time.time())
+        data = [
+            {"id": name, "object": "model", "created": created, "owned_by": "meshnet"}
+            for name in server.model_presets
+        ]
+        self._send_json(200, {"object": "list", "data": data})
+
     def _handle_register(self):
         server: _TrackerHTTPServer = self.server  # type: ignore[assignment]
         body = self._read_json_body()