fix(otelrecv): set close_connection=True so server actually closes after response

Sending Connection: close in the header without closing the server-side
socket left both dagger's Go HTTP client and Python's HTTPServer waiting
for the other to send FIN first. This blocked dagger's OTLP exporter
shutdown, which in turn blocked dagger from exiting.

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

ci/otelrecv.py

@@ -118,6 +118,7 @@ class _Handler(BaseHTTPRequestHandler):
     protocol_version = "HTTP/1.1"
 
     def _respond(self, code, body=b""):
+        self.close_connection = True  # actually close after response, matching the header
         self.send_response(code)
         self.send_header("Content-Type", "application/x-protobuf")
         self.send_header("Content-Length", str(len(body)))