feat(service): mark GetStatus, GetStatuses, ListServices as NO_SIDE_EFFECTS

Annotate the three read-only methods on service.v1.ServiceManager with
idempotency_level = NO_SIDE_EFFECTS so Connect-RPC handlers accept
HTTP GET for them. The service plugin's read endpoints can then sit
behind CDN/proxy caches and be probed from cacheable URLs.

CreateService, Terminate, Restart stay POST-only — each mutates the
service plugin's process table.

Additive proto change; existing POST clients keep working.

Author: rustatian

roadrunner/api/service/v1/service_rpc.proto

@@ -15,7 +15,13 @@ service ServiceManager {
   rpc CreateService(Create) returns (Response);
   rpc Terminate(Service) returns (Response);
   rpc Restart(Service) returns (Response);
-  rpc GetStatus(Service) returns (Status);
-  rpc GetStatuses(Service) returns (Statuses);
-  rpc ListServices(Service) returns (List);
+  rpc GetStatus(Service) returns (Status) {
+    option idempotency_level = NO_SIDE_EFFECTS;
+  }
+  rpc GetStatuses(Service) returns (Statuses) {
+    option idempotency_level = NO_SIDE_EFFECTS;
+  }
+  rpc ListServices(Service) returns (List) {
+    option idempotency_level = NO_SIDE_EFFECTS;
+  }
 }