Java 25 parser: don't merge /// blocks across blank lines

[timtebeek]

Summary

Per JEP 467, a blank line terminates a markdown doc comment, so two /// blocks separated by an empty line are distinct doc comments. The merging logic in formatWithMarkdownJavaDoc previously walked backward through every consecutive /// line without checking the inter-comment suffix, which scrambled the contents when the source was re-printed.

Reproducer (now covered by a TCK test):

/// First doc-comment block.

/// Second doc-comment block.
class Test {}

Before the fix, the source round-tripped to:

/// Secon d doc-comme nt blo
///ck.cond doc-comment block.
class Test {}

The fix is small: when walking the consecutive-/// run backward, break when the previous comment's suffix contains more than one newline (i.e. a blank line gap).

• Found while running FindParseFailures against JabRef/jabref for "There were problems parsing" should show line number and/or excerpt of source code #7554. The varargs/@formatter:off case from that same analysis is already covered by Fix Java parser for annotated varargs by adding marker on J.ArrayType, as opposed to field on J.VariableDeclarations #6098, so this PR only addresses the markdown-doc-comment regression.

Test plan

• New TCK test JavadocTest.MarkdownDocComment.twoTripleSlashBlocksSeparatedByBlankLine fails before the fix and passes after.
• Full :rewrite-java-25:compatibilityTest suite still green.