Build and test on Java 25 with rewrite-java-25

[timtebeek]

What

Migrate the build and test suite to a Java 25 toolchain using the rewrite-java-25 parser (replacing rewrite-java-21), mirroring how openrewrite/rewrite-migrate-java does it. This unblocks recipes/tests that rely on Java 22+ language features (e.g. unnamed _ variables).

Changes

• build.gradle.kts: rewrite-java-21 → rewrite-java-25, and override the toolchain to 25. The org.openrewrite.build.recipe-library plugin pins the toolchain to 21, so swapping the dependency alone is not enough.
• .github/workflows/ci.yml: pass java_version: 25 / 21 to the shared ci-gradle.yml, mirroring rewrite-migrate-java, so CI provisions JDK 25 (the toolchain has no auto-download resolver configured).

Fallout fixed

Running the full suite on JDK 25 surfaced exactly three failures, all pre-existing java-21 → java-25 parser attribution differences (no recipe-logic behavior change):

1. AssertToAssertionsTest.migratesAssertStatementsWithMissingTypeInfo — the java-25 parser leaves assertNotNull(UnknownType.unknownMethod()) unattributed (the erroneous argument prevents overload resolution), so both the type-based matcher and ChangeMethodTargetToStatic miss. Added a fallback in AssertToAssertions that swaps the statically-imported org.junit.Assert.<m> for org.junit.jupiter.api.Assertions.<m> when the call has no method type.
2. MockitoWhenOnStaticToMockStaticTest.shouldShareSingleMockedStaticForConsecutiveStubbingsOfSameClass — the still-used import static org.mockito.Mockito.mockStatic was dropped (non-compiling). The recipe generates the mockStatic call, so the import is now added unconditionally (onlyIfReferenced=false).
3. JMockitMockUpToMockitoTest.mockUpMultipleTest — dropped still-used mock / AdditionalAnswers.delegatesTo imports (non-compiling). Root cause: the mocked Foo/Bar were defined as separate sources, so their types weren't on the JavaTemplate parser classpath; java-25 (stricter than java-21) then left the generated mock/mockConstructionWithAnswer/delegatesTo calls unattributed, and the composite's RemoveUnusedImports unfolded the Mockito.* star import and stripped the null-typed imports. The recipe output was already correct — the fix defines Foo/Bar as nested classes (mirroring the passing mockUpStaticMethodTest's MyClazz) so the types resolve. Verified in isolation that java-25 attributes these calls fine for any classpath-resolvable type.

Verification

Full suite green on JDK 25 — 1731 tests, 0 failures.

Follow-up

• Once this lands on main, PR MockitoWhenOnStaticToMockStatic: name unnamed _ variable when reusing MockedStatic #1060 (fix for MockitoWhenOnStaticToMockStatic fails when using unnamed var (_) notation #1051) should be rebased and reduced to just the recipe fix + its tests, since it currently carries the same build.gradle.kts bump.