containerapp ACR domain suffix hardcoded to .azurecr.io, breaking sovereign cloud support

### Describe the bug

The `containerapp` command module (both in `azure-cli` core and the `containerapp` extension) hardcodes the ACR login server domain suffix as `".azurecr.io"`. This breaks all ACR-related operations when using **Azure sovereign clouds** (e.g., Azure China / Mooncake), where ACR registries use the domain suffix `".azurecr.cn"`.

There are **three root-cause locations**:

---

### Location 1 — Constant definition (hardcoded string)

**File:** `src/azure-cli/azure/cli/command_modules/containerapp/_constants.py`

```python
ACR_IMAGE_SUFFIX = ".azurecr.io"
```

Same constant also duplicated in the extension:

**File:** `src/containerapp/azext_containerapp/_constants.py`

```python
ACR_IMAGE_SUFFIX = ".azurecr.io"
```

---

### Location 2 — `_infer_acr_credentials()` gates all ACR detection

**File:** `src/azure-cli/azure/cli/command_modules/containerapp/_utils.py`

```python
def _infer_acr_credentials(cmd, registry_server, disable_warnings=False):
# If registry is Azure Container Registry, we can try inferring credentials
if ACR_IMAGE_SUFFIX not in registry_server:
raise RequiredArgumentMissingError(
'Registry username and password are required if not using Azure Container Registry.'
)
```

When `registry_server = "myacr.azurecr.cn"`, the check `".azurecr.io" not in registry_server` is `True`, so the function immediately raises an error and treats the China ACR as a **non-ACR registry**. Credential inference, managed identity setup, and `acrpull` role assignment all fail as a result.

---

### Location 3 — `create_acrpull_role_assignment()` crashes with `ValueError`

**File:** `src/azure-cli/azure/cli/command_modules/containerapp/_utils.py`

```python
def create_acrpull_role_assignment(cmd, registry_server, ...):
...
acr_id = acr_show(cmd, client, registry_server[: registry_server.rindex(ACR_IMAGE_SUFFIX)]).id
```

`str.rindex(".azurecr.io")` raises an unhandled `ValueError` when `registry_server` is `"myacr.azurecr.cn"`, causing a crash instead of a clean error message.

### Related command

```bash
az containerapp create \
--name myapp \
--resource-group myRG \
--environment myEnv \
--image myacr.azurecr.cn/myimage:latest \
--registry-server myacr.azurecr.cn

az containerapp update \
--name myapp \
--resource-group myRG \
--image myacr.azurecr.cn/myimage:latest \
--registry-server myacr.azurecr.cn
```

### Errors

Command:
```bash
az containerapp create --name xxx --resource-group xxx --environmentxxx --image xxx.azurecr.cn/xxx/xxx:latest --target-port 80 --cpu 0.5 --memory 1Gi --min-replicas 1 --max-replicas 1 --user-assigned xxx --registry-identity xxx --registry-server xxx.azurecr.cn
```
Error:
```
Usage error: --registry-server, --registry-password and --registry-username are required together if not using Azure Container Registry
```

If we remove --registry-server from the command in hoping it can work around the azurecr.io requirement
```
--registry-identity: expected an ACR registry (*.azurecr.io) for --registry-server
```

### Issue script & Debug output

```bash
az cloud set --name AzureChinaCloud
az login

az containerapp create \
--name myapp \
--resource-group myRG \
--environment myEnv \
--image myacr.azurecr.cn/myimage:latest \
--registry-server myacr.azurecr.cn \
--registry-identity system \
--debug
```

### Expected behavior

The extension should dynamically resolve the ACR domain suffix from the active cloud endpoint configuration rather than hardcoding it.

### Environment Summary

azure-cli 2.84.0

core 2.84.0
telemetry 1.1.0

Extensions:
containerapp 1.3.0b4
resource-graph 2.1.1

Dependencies:
msal 1.35.0b1
azure-mgmt-resource 24.0.0

Python location 'C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\python.exe'
Config directory 'C:\Users\xx\.azure'
Extensions directory 'C:\Users\xxx\.azure\cliextensions'

Python (Windows) 3.13.11 (tags/v3.13.11:6278944, Dec 5 2025, 16:17:02) [MSC v.1944 32 bit (Intel)]

Legal docs and information: aka.ms/AzureCliLegal

Your CLI is up-to-date.

### Additional context

_No response_