Move the core IT suite off the JUnit 4 assertions (4.0.x)

[slachiewicz]

Five classes in its/core-it-suite were already annotated with the Jupiter @Test but still called org.junit.Assert, which is the only thing still pulling JUnit 4 onto the IT classpath. They now use org.junit.jupiter.api.Assertions.

Four needed nothing but the static import swapped. The fifth is the one worth a second look:

-        assertTrue("Jar output path was not built", Files.isRegularFile(jarPath));
+        assertTrue(Files.isRegularFile(jarPath), "Jar output path was not built");

The message is the first argument in JUnit 4 and the last in JUnit 5. Left as a plain import swap it would still compile — assertTrue(String, boolean) has no Jupiter overload, so that one actually fails to compile, but the equivalent assertEquals(message, expected, actual) form does not, and silently asserts the wrong thing. Worth watching for in similar cleanups.

Verification — please read before merging

I could not run this suite. The its reactor needs the 0.1-stub-SNAPSHOT plugin artifacts and 2.1-SNAPSHOT it-support artifacts built first, and that build does not complete in my environment. What I did instead:

• a standalone javac compile check of the changed files against a classpath assembled from the cached maven-it-helper jar plus junit-jupiter-api/opentest4j/apiguardian, with no JUnit 4 jar present — clean
• confirmed no org.junit.Assert reference remains anywhere in its/core-it-suite/src/test/java
• confirmed no line exceeds 120 characters (the its reactor does not configure spotless, so there is no formatter to run)

So this is compile-checked, not test-run. Given the change is import-level plus one argument reorder, CI should settle it — but I would rather say that plainly than imply I ran the ITs.

The same change is on the 4.0.x line as a separate PR.

Generated-by: Claude Opus 5 (1M context)

[gnodet]

Clean and correct JUnit 4 to JUnit 5 assertion migration across 5 integration test files (4.0.x backport). The assertTrue argument reorder in MavenITmng7772CoreExtensionFoundTest is properly handled (JUnit 4 assertTrue(String, boolean) → JUnit 5 assertTrue(boolean, String)). The remaining 4 files are straightforward import swaps with no semantic changes. CI is green across all completed checks.

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Claude Code on behalf of gnodet