#### What I did

On DPU platforms (pensando, nvidia-bluefield), the `routeCheck` monit service is removed from the monit config (sonic-net/sonic-buildimage#26214) since route checking is not applicable on DPUs. However, `config reload` unconditionally tries to `monit unmonitor/monitor routeCheck`, causing:
```
Disabling container and routeCheck monitoring ...
There is no service named "routeCheck"
```

Added a `_monit_service_exists()` helper and guarded `routeCheck` monit calls in `_stop_services()` and `_restart_services()` so they only execute when the service is present in monit.

#### How I did it

- Added `_monit_service_exists(service)` helper that runs `sudo monit summary <service>` and returns `True`/`False` based on exit code.
- In `_stop_services()`: only call `monit unmonitor routeCheck` if `_monit_service_exists("routeCheck")` returns `True`.
- In `_restart_services()`: only call `monit monitor routeCheck` and `_wait_for_monit_service_monitored("routeCheck")` if the service exists.

#### How to verify it

1. On a DPU platform (nvidia-bluefield / pensando) with routeCheck removed from monit config:
 - Run `config reload -y` — should complete without `"There is no service named routeCheck"` error
2. On a non-DPU platform where routeCheck exists in monit:
 - Run `config reload -y` — routeCheck monitoring should still be disabled/re-enabled as before

#### Previous command output (if the output of a command-line utility has changed)

On DPU with routeCheck removed from monit:
```
root@sonic-dpu-3:/home/admin# config reload -y
Acquired lock on /etc/sonic/reload.lock
Disabling container and routeCheck monitoring ...
There is no service named "routeCheck"
Released lock on /etc/sonic/reload.lock
```

#### New command output (if the output of a command-line utility has changed)

On DPU with routeCheck removed from monit:
```
root@sonic-dpu-3:/home/admin# config reload -y
Acquired lock on /etc/sonic/reload.lock
Disabling container monitoring ...
...
Enabling container monitoring ...
Released lock on /etc/sonic/reload.lock
```

Companion PR: sonic-net/sonic-buildimage#26214
Fixes: sonic-net/sonic-buildimage#26225

Signed-off-by: Sonic Build Admin <sonicbld@microsoft.com>