Configure jvmTest to use the JUnit Platform in kmp-module

[alexander-yevsyukov]

What

Configure the jvmTest task in the kmp-module convention plugin to run on
the JUnit Platform (useJUnitPlatform()) and to apply the shared test-logging
setup (configureLogging()).

Why

The kmp-module convention never applied module-testing, and it could not:
module-testing brings the java-library plugin, which conflicts with the
Kotlin Multiplatform plugin kmp-module is built on. As a result the KMP
jvmTest task was left without useJUnitPlatform().

Without it, Gradle falls back to the legacy JUnit-4 test detector, which
discovers none of a module's JUnit 5 / Kotest *Spec.kt classes. A test
task that discovers nothing still passes — so, for example, the core logging
module's jvmTest reported success in ~1 s while running zero of its 13
*Spec.kt suites. This silent gap was being worked around module-locally in
consumer repositories (e.g. an otel-backend useJUnitPlatform() block); the
proper fix belongs in the shared convention here, which also lets those local
workarounds be deleted.

How

Mirror module-testing's test-task configuration onto KMP's jvmTest task:

named<Test>("jvmTest") {
    useJUnitPlatform()
    configureLogging()
}

Unlike module-testing, no includeEngines("junit-jupiter") filter is
imposed: KMP test dependencies include Kotest.runnerJUnit5Jvm, a JUnit
Platform engine of its own, and filtering to Jupiter alone would silently drop
Kotest-native tests — re-creating the bug in a subtler form. This rationale is
captured in the task block's KDoc.

Verification

• ./gradlew -p buildSrc build — PASS (compile, validatePlugins, and
  buildSrc tests all green under JDK 17).
• Reviewers (spine-code-review, kotlin-engineer, review-docs) — all
  APPROVE, no Must-fix / Should-fix.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the kmp-module convention plugin to ensure JVM tests in Kotlin Multiplatform modules are executed on the JUnit Platform and receive the standard shared test logging configuration, closing a gap where JUnit 5 / Kotest tests could be silently undiscovered.

Changes:

• Configure the jvmTest Gradle Test task with useJUnitPlatform().
• Apply shared test logging via configureLogging() to jvmTest.
• Document why module-testing can’t be applied and why no engine filter is used (to avoid excluding Kotest’s JUnit Platform engine).