fix(check-health): retry on unreachable reference endpoint

When multicurl fails to reach the reference RPC (timeout/unreachable),
clean up the temp file, retry up to MAX_RETRIES, then exit with
'unverified (reference unreachable)' instead of falling through as success.

Also add Robinhood mainnet official RPC to reference-rpc-endpoint.json.

Co-authored-by: Cursor <cursoragent@cursor.com>

reference-rpc-endpoint.json

@@ -3394,6 +3394,7 @@
     "lagging_lag": 20,
     "syncing_lag": 40,
     "urls": [
+      "https://rpc.mainnet.chain.robinhood.com",
       "https://robinhood.drpc.org"
     ]
   },