App Service site name truncation (`take(name-uniqueString, 60)`) can weaken uniqueness for long resource names

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

App Service website resources generated for `AddAzureAppServiceEnvironment` name each site using:

```bicep
name: take('${toLower('api')}-${uniqueString(resourceGroup().id)}', 60)
```

The `uniqueString(resourceGroup().id)` disambiguator (13 chars) is placed at the **end** of the string, and `take(..., 60)` truncates from the **tail**. For short resource names this is fine, but when the Aspire resource name is long (roughly ≥46 characters), the `take(…, 60)` truncation eats into — or entirely removes — the trailing `uniqueString`. Two app services whose names share a long common prefix (differing only after the truncation boundary) can then truncate to the **same** 60-character site name, causing a resource-name collision in the same resource group.

This is the same class of truncation-vs-uniqueness problem previously seen with Azure Container App environment naming: putting the uniqueness suffix in a position where it can be truncated defeats its purpose.

Note: this is a **pre-existing** naming pattern in the App Service environment publisher. It was surfaced while reviewing #18738 (delegated subnet support) but is **not** introduced or changed by that PR — the delegated-subnet additions (shared VNet-level subnet, and the per-site `networkConfig` child namespaced under the unique site name) do not add collision risk. Filing separately so the underlying site-name truncation weakness is tracked.

### Expected Behavior

Site names should preserve the `uniqueString` disambiguator even when the resource name is long — e.g. by truncating the name prefix while guaranteeing the `uniqueString` suffix is always retained, so two distinct app services can never collapse to the same site name.

### Steps To Reproduce

Inspect the generated Bicep for an App Service website with a long resource name, or the snapshot `tests/Aspire.Hosting.Azure.Tests/Snapshots/AzureAppServiceTests.AddAppServiceWithDelegatedSubnet.verified.bicep` (site name on the `webapp` resource). Two app services whose names share a ≥46-character prefix would `take(…, 60)` to the same value.

### Anything else?

- Affected area: `Aspire.Hosting.Azure.AppService` site name generation.
- Related PR (context only, not the cause): https://github.com/microsoft/aspire/pull/18738
- Only the site-name generation is affected. Child resources (`main`, `stage`, `virtualNetwork` networkConfig, `slotConfigNames`) inherit the unique parent site name as their namespace and are safe.

Start in the Aspire.Hosting.Azure.AppService site-name generation and inspect the generated Bicep in tests/Aspire.Hosting.Azure.Tests/Snapshots/AzureAppServiceTests.AddAppServiceWithDelegatedSubnet.verified.bicep. Add coverage for long resource names and verify that generated site names retain the uniqueString suffix and remain distinct after truncation.

Written by the indexing model from the issue text.