[PLAT-18888] Otel process is not disabled after stopping metrics/logs export

Summary:
Otel process is not disabled after stopping metrics/logs export.
This diff fixes the otel install, start and enable process via node agent.
Before this diff, the otel collector was not being stopped and disabled correctly. Noticed it also needs a fix to start and enable node agent.

Before this diff (Notice it is active but disabled):
```
#After enabling audit logs, metric export, or both
[yugabyte@yb-admin-sahith-uni-2-n1 ~]$ systemctl --user status otel-collector
● otel-collector.service - OpenTelemetry Collector
     Loaded: loaded (/home/yugabyte/.config/systemd/user/otel-collector.service; disabled; preset: disabled)
     Active: active (running) since Wed 2025-11-05 05:09:11 UTC; 17s ago
   Main PID: 820726 (otelcol-contrib)
      Tasks: 6 (limit: 21916)
     Memory: 57.3M (max: 2.0G available: 1.9G)
        CPU: 359ms
     CGroup: /user.slice/user-1994.slice/[email protected]/app.slice/otel-collector.service
             └─820726 /home/yugabyte/otel-collector/otelcol-contrib --config=file:/home/yugabyte/otel-collector/config.yml
```

After this diff (active + enabled or inactive + disabled, which is correct and expected):
```
#After enabling audit logs, metric export, or both
[yugabyte@yb-admin-sahith-uni-2-n1 ~]$ systemctl --user status otel-collector
● otel-collector.service - OpenTelemetry Collector
     Loaded: loaded (/home/yugabyte/.config/systemd/user/otel-collector.service; enabled; preset: disabled)
     Active: active (running) since Wed 2025-11-05 05:18:52 UTC; 10s ago
   Main PID: 826541 (otelcol-contrib)
      Tasks: 6 (limit: 21916)
     Memory: 66.4M (max: 2.0G available: 1.9G)
        CPU: 381ms
     CGroup: /user.slice/user-1994.slice/[email protected]/app.slice/otel-collector.service
             └─826541 /home/yugabyte/otel-collector/otelcol-contrib --config=file:/home/yugabyte/otel-collector/config.yml

# After disabling both audit logs and metrics export
[yugabyte@yb-admin-sahith-uni-2-n1 ~]$ systemctl --user status otel-collector
○ otel-collector.service - OpenTelemetry Collector
     Loaded: loaded (/home/yugabyte/.config/systemd/user/otel-collector.service; disabled; preset: disabled)
     Active: inactive (dead)

```

Test Plan:
Manually tested by clean compiling, then reinstalling node agent on the universe.
Enabled metric export, disabled metric export.
Enabled audit log export, disabled audit log export.
Enabled DBME and DBAL, disabled only metric export - noticed otel is still running as expected.

Also:
Run UTs.
Run itests.

Reviewers: nsingh

Reviewed By: nsingh

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D48039

Author: Sahith02

managed/node-agent/app/task/install_otel_collector.go

@@ -87,8 +87,13 @@ func (h *InstallOtelCollector) Handle(ctx context.Context) (*pb.DescribeTaskResp
 		return nil, err
 	}
 
-	// 4) stop the systemd-unit if it's running.
-	if err := module.StopSystemdService(ctx, h.username, OtelCollectorService, h.logOut); err != nil {
+	// 4) Stop and disable the systemd service
+	if err := module.DisableSystemdService(
+		ctx,
+		h.username,
+		OtelCollectorService,
+		"", // Don't remove the unit file - we need it for re-enabling later
+		h.logOut); err != nil {
 		return nil, err
 	}
 
@@ -99,9 +104,14 @@ func (h *InstallOtelCollector) Handle(ctx context.Context) (*pb.DescribeTaskResp
 		return nil, err
 	}
 
-	// 6) Start and enable the otel-collector service.
-	if err = module.StartSystemdService(ctx, h.username, OtelCollectorService, h.logOut); err != nil {
-		return nil, err
+	// 6) Start and enable the service only if config file exists
+	if h.param.GetOtelColConfigFile() != "" {
+		if err = module.StartSystemdService(ctx, h.username, OtelCollectorService, h.logOut); err != nil {
+			return nil, err
+		}
+		if err = module.EnableSystemdService(ctx, h.username, OtelCollectorService, h.logOut); err != nil {
+			return nil, err
+		}
 	}
 	return nil, nil
 }