APM agent breaks log.error when using StructuredLogEncoder with ecs format

## Describe the bug
We are using the version 1.55.5 with experimental Spring Boot 4 support. Since the update we have problems with `log.error` in SLF4J. This is the logback-spring.xml configuration:
```xml

ecs

```
When using the apm agent we are seeing this in the logs instead of the real error log:

07:18:54,651 |-ERROR in ch.qos.logback.core.ConsoleAppender[CONSOLE] - Appender [CONSOLE] failed to append. java.lang.IllegalStateException: The name 'error' has already been written

According to my research, this happens if the JSON log already contains a field with the same name, in this case `error`. Logback uses the field `error` for a complex object that contains `error.message` and `error.stack_trace`. This is also documented in the [Elastic Common Schema](https://www.elastic.co/docs/reference/ecs/ecs-error).

## Steps to reproduce
Use the org.springframework.boot.logging.logback.StructuredLogEncoder with ecs format as described and call `org.slf4j.Logger#error(java.lang.String, java.lang.Throwable)`.

## Expected behavior
No exception during logging, but a well-formed JSON log containing the error.