[#3879] Feature/introduce kotlin extension

[jangalinski]

• Import existing extension-kotlin into extensions/kotlin - keeping commit history
• clean up maven setup and use correct parents
• move everything not applicable (directly) to _todo

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Closes AxonFramework/extension-kotlin#393. Takes over #3878 by @jangalinski.

Context

@jangalinski opened #3878 to import the extension-kotlin repository into
extensions/kotlin/ (with full commit history) and wire it into the monorepo build.
That PR intentionally stopped there: incompatible AF4 files were moved to _todo/
rather than rewritten, leaving the actual AF5 migration as follow-up work.

This PR completes that migration. The reference for every change is
extension-kotlin#394,
which performed the same migration against the standalone extension-kotlin
repository (targeting AF 5.1.0).

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Differences from extension-kotlin#394

The monorepo integration required a few adjustments beyond a straight copy of the patch.

extensions/kotlin/pom.xml — parent and coordinates

PR#394 kept the standalone axon-kotlin-parent / 4.12.x-SNAPSHOT parent and
simply updated axon.version to 5.0.3.

Our branch replaces the entire parent with the monorepo coordinates:

• parent → axon-extensions / 5.1.0-SNAPSHOT
• version → 5.1.0-SNAPSHOT (inherited)
• All standalone release/deploy/SCM infrastructure removed (sonatype, gpg, dokka,
  release-plugin, pluginRepositories, scm, developers, .github/, .editorconfig,
  mvnw, coverage-report/, etc.)
• docs/ removed — AF4 Antora reference content imported via history, no AF5
  equivalent yet; reference docs update is a separate effort

Additionally the <log4j.version>2.13.3</log4j.version> property present in the
standalone pom was removed. In the monorepo it overrides the managed 2.25.3 version
and causes a Maven resolution failure (log4j-slf4j2-impl:2.13.3 does not exist in
Maven Central — only introduced in log4j 2.20).

CommandGatewayExtensions.kt — sendAndWait null safety

CommandGateway.sendAndWait(Object, Class<R>) is @Nullable. PR#394 silently trusts
the platform type. Our branch adds ?: error(...) to fail fast with a clear message
pointing callers to sendForResult as the nullable alternative.

QueryUpdateEmitterExtensions.kt — type bound and SAM lambda

Two changes vs PR#394: Q : Any bound added (required to resolve the correct Java
overload), and filter wrapped in an explicit lambda to satisfy Predicate<? super Q>
unambiguously. @Suppress("UNCHECKED_CAST") covers the remaining compiler warning.

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What was done (full summary)

Build (extensions/kotlin/pom.xml)

• Parent changed to monorepo axon-extensions / 5.1.0-SNAPSHOT
• Removed <log4j.version>2.13.3</log4j.version> (would break monorepo resolution)
• Removed AF4 compile dependencies: jackson-module-kotlin,
  kotlinx-serialization-core, javax.xml.bind:jaxb-api
• Removed all-open:annotation=...AggregateRoot compiler plugin option
• axon-configuration → axon-messaging in dependency management

Deleted — AF4-only, no AF5 equivalent (yet)

File	Reason
AggregateLifecycleExtensions.kt	AggregateLifecycle / @AggregateRoot removed in AF5
BuilderExtensions.kt	EventSourcingRepository / GenericJpaRepository builders not present in AF5
ResultDiscriminatorCommandCallback.kt	CommandCallback / CommandResultMessage replaced by CommandResult
EventUpcaster.kt	SingleEventUpcaster not available in AF5.0/5.1; tracked for 5.2
messaging/responsetypes/ArrayResponseType.kt	AbstractResponseType removed; ResponseType hierarchy reworked
serialization/ (entire module)	Serializer replaced by Converter; full rewrite out of scope — tracked as follow-up
META-INF/spring-devtools.properties	Spring Boot DevTools dependency removed
All corresponding tests	Deleted with their production classes

Rewritten for AF5

CommandGatewayExtensions.kt — send, sendForResult, sendAndWait over
CommandResult<R> (replaces AF4 CommandCallback / CommandResultMessage pattern).

QueryGatewayExtensions.kt — query, queryMany, subscriptionQuery retained;
scatterGather* replaced by streamingQuery; type bound R : Any on all functions
where R is used as Class<R>.

QueryUpdateEmitterExtensions.kt — package import updated to AF5;
Q : Any bound added; explicit SAM lambda for Predicate.

kotlin-test/FixtureExtensions.kt — AggregateTestFixture / SagaTestFixture /
TestExecutor / ResultValidator replaced by the AF5 phase-based
AxonTestFixture / AxonTestPhase API; whenever(), whenever(command),
exception(KClass), exception(KClass, message) rewritten accordingly.

Tests

18 tests pass (12 in kotlin, 6 in kotlin-test). Test files are identical to
the ones in PR#394.

Documentation

README.md: versions 4.6.0 → 5.1.0-SNAPSHOT, CI/SonarCloud/CodeTriage badges
removed (standalone-repo artefacts), issue tracker URL updated to monorepo,
docs profile name corrected (javadoc-and-sources → docs-and-sources).

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Out of scope / follow-up

Item	Notes
KotlinSerializer / KotlinConverter	Requires full rewrite against AF5 Converter API
EventUpcaster DSL	SingleEventUpcaster absent from AF5.0/5.1; scheduled for 5.2

[jangalinski]

Closing this in favor of #4432 (target: axon-5.1.x)