### 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
private boolean lenient;
    public static SourceConfig randomSourceConfig() {
        return new SourceConfig(
            generateRandomStringArray(10, 10, false, false),
            QueryConfigTests.randomQueryConfig(),
            randomRuntimeMappings()
        );
    }
    public static SourceConfig randomInvalidSourceConfig() {
        
        return new SourceConfig(
            generateRandomStringArray(10, 10, false, false),
            QueryConfigTests.randomInvalidQueryConfig(),
            randomRuntimeMappings()
        );
    }
    private static Map<String, Object> randomRuntimeMappings() {
        return randomList(0, 10, () -> randomAlphaOfLengthBetween(1, 10)).stream()
            .distinct()
            .collect(toMap(f -> f, f -> singletonMap("type", randomFrom("boolean", "date", "double", "keyword", "long"))));
    }
    @Before
    public void setRandomFeatures() {
        lenient = randomBoolean();