fix: validate connection string before starting the broker process

Addresses review feedback on #1245. In mean_time_recover_with_docker.py the
environment check sat after process_docker.start() and a five second sleep, so
an unset IOTHUB_DEVICE_CONNECTION_STRING raised only once the container restart
experiment was already running, leaving the parent waiting on a live non-daemon
child. The check now runs first thing in main(), before the certificate is read
and before any process is spawned, so the script fails immediately with the
clear error.

simple_send_message.py already validated before any side effect and is
unchanged.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ewertons

sdklab/meantimerecovery/mean_time_recover_with_docker.py

@@ -125,6 +125,14 @@ def timer_done():
 
 async def main():
 
+    conn_str = os.getenv("IOTHUB_DEVICE_CONNECTION_STRING")
+    if not conn_str:
+        raise RuntimeError(
+            "Set the IOTHUB_DEVICE_CONNECTION_STRING environment variable to the connection "
+            "string of the device used against the local test broker, e.g. "
+            "HostName=localhost;DeviceId=devicemtr;SharedAccessKey=<base64 key>"
+        )
+
     ca_cert = "self_cert_localhost.pem"
     certfile = open(ca_cert)
     root_ca_cert = certfile.read()
@@ -141,13 +149,6 @@ async def main():
 
     # Do not delete sleep from here. Server needs some time to start.
     await asyncio.sleep(5)
-    conn_str = os.getenv("IOTHUB_DEVICE_CONNECTION_STRING")
-    if not conn_str:
-        raise RuntimeError(
-            "Set the IOTHUB_DEVICE_CONNECTION_STRING environment variable to the connection "
-            "string of the device used against the local test broker, e.g. "
-            "HostName=localhost;DeviceId=devicemtr;SharedAccessKey=<base64 key>"
-        )
     device_client = IoTHubDeviceClient.create_from_connection_string(
         conn_str, keep_alive=KEEP_ALIVE, server_verification_cert=root_ca_cert
     )