JsonParseException when YAML mapping key is a sequence

# JsonParseException when YAML mapping key is a sequence

## Description

I am trying to parse a YAML document using Jackson’s `YAMLMapper` (jackson-dataformat-yaml) in Kotlin/Java, but parsing fails when a mapping key is a sequence. The exception occurs **before any schema validation**, indicating that `YamlMapper` cannot handle this case.

## YAML Example

```yaml
? [user, 123]
: name: Иван
age: 30
```

## Kotlin Example

```kotlin
import com.fasterxml.jackson.dataformat.yaml.YAMLMapper

fun main() {
val yaml = """
? [user, 123]
: name: Иван
age: 30
""".trimIndent()

val mapper = YAMLMapper()
val node = mapper.readTree(yaml) // Exception occurs here
}
```

## Exception

```
com.fasterxml.jackson.core.JsonParseException:
Expected a field name (Scalar value in YAML), got this instead:

```

## Environment

* jackson-dataformat-yaml version: 2.15.x
* Kotlin/Java 17
* OS: macOS / Linux

## Notes / Expected Behavior

* According to the YAML spec, using a sequence as a mapping key is valid.
* Jackson fails because `JsonNode` only supports scalar keys.
* Ideally, Jackson should either parse the key into a string representation or provide a clear API to handle non-scalar mapping keys.

## References

* YAML spec: [https://yaml.org/spec/1.2/spec.html#id2760799](https://yaml.org/spec/1.2/spec.html#id2760799)
* Jackson YAMLMapper / YAMLFactory

## Request

* Support for sequence (and possibly other complex structures) as mapping keys, or
* Better documentation / error message when sequence is used as a key.