[fixes #4051] Eclipse isClass must not treat non-type nodes as classes

[kamilkrzywanski]

Summary

Fixes #4051: Eclipse/VSCode autocompletion crashed with ClassCastException: CompletionOnFieldType cannot be cast to TypeDeclaration in HandleValue (and potentially other type-level handlers).

Root cause: EclipseHandlerUtil.isTypeAndDoesNotHaveFlags (used by isClass(), isClassOrEnum(), …) returned true when the node was not a TypeDeclaration. Non-type nodes (including JDT's CompletionOnFieldType, which extends FieldDeclaration) got modifiers = 0, so (0 & flags) == 0 was true. Handlers then unguardedly cast to TypeDeclaration.

Fix: Match javac's JavacHandlerUtil.isClassAndDoesNotHaveFlags — return false immediately when the node is not a TypeDeclaration.

This is a single-point root-cause fix that protects all handlers relying on isClass / related helpers, not a HandleValue-only band-aid.

Reproducers / tests

1. Unit test TestEclipseIsClass — fails on the old isClass(fieldNode) == true behavior; passes after the fix. Wired into TestEclipse (CI) and RunCoreTests.
2. Transform tests for the reported scenario:
   • @Builder + @Jacksonized on an inner class of a sealed interface
   • same with @Value (the stacktrace path through HandleValue)

Compile-path (javac / mvn compile) already succeeded, as the issue stated; the bug is in the JDT completion AST path.

Test plan

• TestEclipseIsClass fails before fix, passes after
• ant test.javacCurrent — delombok transform tests for the new sealed-interface cases
• ant test.eclipse-202503 — ECJ transform tests for the same cases
• Full local ant test (javacCurrent + eclipse-202503)
• Full CI on this PR

Note: pre-existing ValUndenotable ECJ failure on master

Local full ant test reports one ECJ failure that is not related to this PR:

[FAIL] ecj-ValUndenotable.java
Missing / empty expected file: ValUndenotable.java

• test/transform/resource/before/ValUndenotable.java and after-delombok/ValUndenotable.java exist on master
• after-ecj/ValUndenotable.java is missing on master as well (introduced with the JDK26/var work, without an ECJ golden file)

A/B check: temporarily reverted this PR's isTypeAndDoesNotHaveFlags change back to the master implementation and re-ran the ECJ suite:

	Without this fix	With this fix
TestEclipseIsClass (field node)	FAIL (expected)	PASS
ecj-ValUndenotable	FAIL — missing after-ecj file	FAIL — same missing file, same actual output

So ValUndenotable fails identically with or without this change; it is a pre-existing missing golden file, not a regression from #4051.