Improve implementations of LogEvent.toImmutable() and ReusableMessage.memento()

[ppkarwasz]

The implementations of LogEvent.toImmutable() either use serialization or one of the Log4jLogEvent.Builder constructors and custom per class code.

This PR:

• Redirects all implementation of LogEvent.toImmutable to the Builder(LogEvent) constructor (except Log4jLogEvent.toImmutable())
• Improve the constructor to create really immutable and thread-safe instances.
• Removes the usage of ThrowableProxy for purposes not related to serialization.
• Fixes some implementations of ReusableMessage.memento().
• Uses ReusableMessage.memento() instead of ImmutableMessage where applicable.

[Copilot]

Pull Request Overview

This PR improves the implementations of LogEvent.toImmutable() and ReusableMessage.memento() by unifying the immutable conversion through the Log4jLogEvent.Builder, removing deprecated and unnecessary thrownProxy usages, and ensuring that messages are properly cached in their immutable forms.

• Redirects all implementations of LogEvent.toImmutable() to use the Builder(LogEvent) constructor
• Updates memento() implementations in reusable messages to cache their formatted strings
• Removes thrownProxy usage from non‐serialization contexts and adapts tests accordingly

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
log4j-core/impl/ReusableLogEventFactory.java	Updates in Javadoc and thread info retrieval to clarify message mutation semantics.
log4j-core/impl/MutableLogEvent.java	Replaces createMemento() with toImmutable() and removes thrownProxy references.
log4j-core/impl/Log4jLogEvent.java	Refactors immutability, lazy initialization, and thrownProxy handling; adjusts equals/hashCode.
log4j-core/config/LoggerConfig.java	Removes deprecated LogEvent instantiation in property evaluation.
log4j-core/async/*	Uses toImmutable() consistently for converting mutable events.
log4j-api/message/*	Updates memento() implementations to cache formatted messages.
log4j-1.2-api/rewrite/*	Simplifies event rewrite by removing thrownProxy setting.
Test files	Update assertions to check for immutable messages instead of legacy memento types.

Comments suppressed due to low confidence (1)

log4j-core/src/main/java/org/apache/logging/log4j/core/impl/Log4jLogEvent.java:1091

• [nitpick] Removing thrownProxy from the equals and hashCode methods alters the equality semantics; please verify that this change does not break any contract or expected behavior.

if (thrownProxy != null ? !thrownProxy.equals(that.thrownProxy) : that.thrownProxy != null) {