### Elasticsearch Version

main

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

n/a

### Problem Description

This requires further verification, but I suspect comma-separated string of source index name in transform configuration:
```
"source": {
  "index": "my-index*,my-other-index*"
}
```

may not resolve properly and needs to be replaced with:
```
"source": {
  "index": ["my-index*","my-other-index*"]
}
```

We should make both syntaxes work equally fine.

### Steps to Reproduce

Provide comma-separated list of indices in the `"source"` section of transform config:
```
"source": {
  "index": "my-index*,my-other-index*"
}
```

### Logs (if relevant)

_No response_
x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/transform/transforms/SourceConfigTests.java
emptyMap()
        );
        assertThat(sourceConfig.getRuntimeMappings(), is(anEmptyMap()));
        assertThat(sourceConfig.getScriptBasedRuntimeMappings(), is(anEmptyMap()));
    }
    public void testGetRuntimeMappings_NonEmptyRuntimeMappings() {
        Map<String, Object> runtimeMappings = new HashMap<>() {
            {
                put("field-A", singletonMap("type", "keyword"));
                put("field-B", singletonMap("script", "some script"));
                put("field-C", singletonMap("script", "some other script"));
            }
        };
        Map<String, Object> scriptBasedRuntimeMappings = new HashMap<>() {
            {
                put("field-B", singletonMap("script", "some script"));
                put("field-C", singletonMap("script", "some other script"));
            }
        };
        SourceConfig sourceConfig = new SourceConfig(
            generateRandomStringArray(10, 10, false, false),
            QueryConfigTests.randomQueryConfig(),
            runtimeMappings
        );
        assertThat(sourceConfig.getRuntimeMappings(), is(equalTo(runtimeMappings)));
        assertThat(sourceConfig.getScriptBasedRuntimeMappings(), is(equalTo(scriptBasedRuntimeMappings)));
    }
    public void testRequiresRemoteCluster() {
        assertFalse(
            new SourceConfig(new String[] { "index1", "index2", "index3" }, QueryConfigTests.randomQueryConfig(), randomRuntimeMappings())