Restore default private_key value for backward compatibility

Keep privateKeyFile.pem as the default so existing users with
separate cert/key file configs are not affected.

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

application/src/main/java/org/thingsboard/server/service/edge/rpc/EdgeGrpcService.java

@@ -113,7 +113,7 @@ public class EdgeGrpcService extends EdgeRpcServiceGrpc.EdgeRpcServiceImplBase i
     private boolean sslEnabled;
     @Value("${edges.rpc.ssl.cert}")
     private String certFileResource;
-    @Value("${edges.rpc.ssl.private_key:}")
+    @Value("${edges.rpc.ssl.private_key}")
     private String privateKeyResource;
     @Value("${edges.rpc.ssl.key_password:}")
     private String keyPassword;

application/src/main/resources/thingsboard.yml

@@ -1491,7 +1491,7 @@ edges:
       # For a combined PEM file containing both the certificate chain and the private key, leave private_key empty.
       cert: "${EDGES_RPC_SSL_CERT:certChainFile.pem}"
       # Path to the private key PEM file. Optional when the key is already included in the cert file (combined PEM).
-      private_key: "${EDGES_RPC_SSL_PRIVATE_KEY:}"
+      private_key: "${EDGES_RPC_SSL_PRIVATE_KEY:privateKeyFile.pem}"
       # Password for encrypted private keys. Optional, only needed for password-protected keys.
       key_password: "${EDGES_RPC_SSL_KEY_PASSWORD:}"
     # Maximum size (in bytes) of inbound messages the cloud can handle from the edge. By default, it can handle messages up to 4 Megabytes