Cloud SRE Agent App - Support custom entities (CUSTOM_DEVICE) with optional ARN

**Describe the bug**
Custom entities (e.g. `CUSTOM_DEVICE-*`) are not handled by the app. Some carry an ARN
that ties them to a cloud provider; these should be routable to the matching agent the
same way native AWS/Azure/GCP entities are.

Example entity:
{
"entity.name": "edu-cluster",
"id": "CUSTOM_DEVICE-7857C3B822059692",
"arn": "arn:aws:ecs:us-east-1:135808943972:cluster/edu-cluster",
"tags": [
"location:us-east-1",
"[AWS]env:stg"
]
}

Custom entities without an ARN also exist and need a fallback (e.g. infer provider from
tags, or skip).

Start by tracing the existing AWS, Azure, and GCP entity-routing entry points used by the Cloud SRE Agent App. Reproduce routing with the CUSTOM_DEVICE example and inspect how custom entities without an ARN are represented; done means custom entities with provider ARNs route correctly and the no-ARN behavior is explicitly defined.

Written by the indexing model from the issue text.