Fix fluent methods on cloned builders

[hutiefang76]

Fixes #2782.

BaseBuilder cached this in a final thisB field. When a builder is cloned, that final field is copied as-is, so fluent methods invoked on the clone still return the original builder instance. This can make chained configuration on cloned builders silently continue on the wrong object.

This keeps the existing self-type helper pattern but makes it resolve this dynamically through one private helper method instead of storing it in a clone-stale field. That avoids the broader refactor from #2783 and keeps the unchecked cast centralized.

Validation:

mvn -pl core -Dtoolchain.skip=true -Dtest=BaseBuilderTest#clonedBuilderFluentMethodsReturnClone test
mvn -pl core -Dtoolchain.skip=true -Dtest=BaseBuilderTest test
git diff --check
mvn -pl core -Dtoolchain.skip=true test

The full core test run passed 635 tests with 0 failures/errors and 3 skipped tests.