Fix AddCommentToXmlTag for self-closing and empty tags

[Jenson3210]

Summary

• Resolves AddCommentToXmlTag doesn't handle self-closing or empty tags #7777: AddCommentToXmlTag silently skipped self-closing tags (<foo/>) and threw IndexOutOfBoundsException on empty tags (<foo></foo>).
• Visitor now derives the comment prefix from the existing first child (or "" when content is null/empty) and always routes through Xml.Tag#withContent, which already materializes a Closing tag when transitioning a self-closing tag to one with content.
• Also fixes a latent bug where the previous implementation mixed tag.getContent() (pre-recursion) with t.withContent(...), dropping nested transformations.

Test plan

• New test: comment is added to a self-closing tag (<foo bar="baz"/> → <foo bar="baz"><!-- hello --></foo>).
• New test: comment is added to an empty tag (<foo></foo> → <foo><!-- hello --></foo>) with no exception.
• New test: idempotency — recipe does not duplicate an existing matching comment.
• Existing addCommentToDependencyBlock test still passes.
• ./gradlew :rewrite-xml:test --tests "org.openrewrite.xml.AddCommentToXmlTagTest" → BUILD SUCCESSFUL.

[timtebeek]

I wonder if we should prepend the comment to the self closing tag instead of adding a new close tag to add the comment within the tag. What are your thoughts on that?

My motivation would be a more minimal diff, with a clear line introduced before the existing line in a well formatted XML document.

[Jenson3210]

I also preferred that, however, that is a bigger change as there is already expectations of comments being added inside the tags.
Would you want to have different behavior when tag is empty vs non-empty?

rewrite/rewrite-xml/src/test/java/org/openrewrite/xml/AddCommentToXmlTagTest.java

Line 29 in 2047043

void addCommentToDependencyBlock() {

[timtebeek]

I'd be fine with slightly different behavior if that leads to less surprises for the self closing case.

[Jenson3210]

implemented this change here.