Fix RemoveAnnotation crash on Groovy trees when rewrite-kotlin is on the classpath

[Jenson3210]

Summary

• PR Kotlin recipe DSL (K2 compiler plugin) + cross-language visitor mixins #7701 introduced the cross-language mixin SPI: language modules can contribute mixins via META-INF/rewrite/mixins/<base-visitor-fqn>. rewrite-kotlin ships RemoveAnnotationKotlinMixin for org.openrewrite.java.RemoveAnnotationVisitor.

The registry is a shared namespace — multiple language modules can register a mixin for the same base visitor. But TreeVisitorAdapter.discoverRegisteredMixin returned the first entry it found without checking it was assignable to the requested adaptTo class. With both rewrite-groovy and rewrite-kotlin on the classpath (e.g. via rewrite-gradle, rewrite-spring, etc.), visiting a Groovy build.gradle did this:

1. G.accept → v.adapt(GroovyVisitor.class)
2. discoverRegisteredMixin returned RemoveAnnotationKotlinMixin (registered by rewrite-kotlin)
3. adapt()'s assignability guard threw:

java.lang.IllegalArgumentException: Mixin org.openrewrite.kotlin.RemoveAnnotationKotlinMixin
is not assignable to org.openrewrite.groovy.GroovyVisitor; the mixin must extend (or be a
subtype of) the adaptTo class so that proxy generation can extend it.

This broke every downstream user that runs RemoveAnnotation (or any Java recipe that ends up adapting through G) against a Groovy build script — reported via SpringBootVersionUpgradeTest.upgradeVersionGradle in rewrite-spring.

Fix

Filter the registry entries inside discoverRegisteredMixin by adaptTo.isAssignableFrom(mixinClass) and continue past mismatched entries. A Kotlin mixin is silently ignored when adapting to GroovyVisitor (and vice versa) while still being picked up when the visit target really is a Kotlin tree.

• rewrite-core/src/main/java/org/openrewrite/internal/TreeVisitorAdapter.java — skip cross-language mixin entries.
• rewrite-gradle/src/test/java/org/openrewrite/gradle/RemoveAnnotationGradleTest.java — regression test: runs RemoveAnnotation against a Groovy build.gradle (in a module where both rewrite-groovy and rewrite-kotlin are on the classpath). Fails on main with the exact stack trace above, passes with the fix.

Test plan

• New RemoveAnnotationGradleTest.doesNotApplyKotlinMixinToGroovyTree fails on main with the reported IllegalArgumentException.
• Same test passes after the fix.
• ./gradlew :rewrite-core:test :rewrite-kotlin:test :rewrite-groovy:test :rewrite-gradle:test (running locally; no Kotlin @file:OptIn removal coverage exists in rewrite-kotlin itself, so the Kotlin mixin path is only covered indirectly by the existing adapter-level tests).
• Downstream repro SpringBootVersionUpgradeTest.upgradeVersionGradle in rewrite-spring should pass once this lands.