[202412] Add FRR failed route check in route_check.py

[ZhaohuiS]

What I did

Cherry-pick of sonic-net/sonic-utilities#4119 to 202412 branch with conflict resolution.

It supports to detect the route with offload False or without offload, which can capture queued route, because queued route doesn't have offload, but for rejected route, the offload is True. It can't detect the rejected route.
So, we add a new detection check for the value of key failed for route entries, which can cover both rejected and queued routes.
It will help to detect rejected route and queued route on device.

How I did it

Append failed route prefix into failed list, if it's not empty, script will print error message into syslog

        # Check for failed state
        if entry.get('failed', False):
            failed_rt.append(route_prefix)

How to verify it

• For rejected route: route_check.py will report failed_FRR_routes in the failure results
• For queued route: route_check.py will report both missed_FRR_routes and failed_FRR_routes

Conflict resolution notes

The 202412 branch of Azure/sonic-utilities.msft has a different code structure from sonic-net/sonic-utilities master:

• Uses get_frr_routes() instead of get_frr_routes_parallel()
• Uses namespace-keyed results dict (results[namespace])
• Test case numbering starts from 35 (last existing case is 34)

All changes were adapted to match the 202412 branch code style.