[CH] Unsupported function overlay and without fallback

### Backend

CH (ClickHouse)

### Bug description

SQL:
```
select overlay(LSTG_FORMAT_NAME PLACING 'abc' FROM 1),
overlay(LSTG_FORMAT_NAME PLACING 'abc' FROM 1 FOR 1)
from test_kylin_fact
```

Error:

Sig[Overlay](OVERLAY) is defined in ExpressionMappings, but there is no function mapping defined in CH backend

### Spark version

None

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

_No response_

Start by inspecting ExpressionMappings and the ClickHouse backend mapping for Overlay. Reproduce the supplied SQL with both overlay forms, then verify that the ClickHouse backend handles the function and its optional FOR clause without the reported unsupported-function error.

Written by the indexing model from the issue text.