Preserve XML comments that auto-format or content removal used to drop

[timtebeek]

• Follow-up to Keep XML comments trailing an element on the same line #8353. That PR made a trailing comment's prefix authoritative by dropping the unconditional line break in LineBreaksVisitor. That line break had also been acting as an accidental repair for visitors that destroy a comment's spacing, so three latent defects are now user-visible. All three are reproduced by tests that fail on main.

1. Auto-format silently deletes anything after the root element

Xml.Document has no trailing-misc list — a comment or processing instruction after </project> is stored verbatim in the eof String. RemoveTrailingWhitespaceVisitor.visitDocument filtered eof down to its \n/\r characters, so every recipe that auto-formats dropped it:

 <project>
   <excludes/>
 </project>
-<!--why project-->

• This is data loss rather than reflow, and it predates Keep XML comments trailing an element on the same line #8353 (the own-line case fails there too); an identity parse/print round-trips the comment fine, so it is the formatter. Now only horizontal whitespace at line ends is stripped, which is equivalent to the old filter for the whitespace-only eof that is the common case.

2. RemoveContentVisitor collapses an orphaned trailing comment

• Called out as a known follow-up in Adopt upstream XML trailing-comment formatting rewrite-migrate-java#1180. The removed element's prefix carried the line break, so the comment that trailed it has none left to inherit:

 <dependency>
     <groupId>group</groupId>
-    <version/><!-- why version -->
+<!-- why version -->
 </dependency>

The removed content's prefix is now transferred to the following sibling when that sibling has no line break of its own. This one is independent of auto-format — it reproduces with RemoveContentVisitor alone.

3. removePrecedingComment eats the previous sibling's trailing comment

• Under Keep XML comments trailing an element on the same line #8353's semantics <!-- why group --> below documents <groupId>, but removing <version/> with removePrecedingComment = true deleted it:

 <dependency>
-    <groupId>group</groupId> <!-- why group -->
+    <groupId>group</groupId>
 </dependency>

Gated on the same predicate LineBreaksVisitor uses: a comment sharing a line with an earlier sibling is that sibling's. A comment on its own line, or one that is the first content of its tag, is still removed.

Tests

New, each failing on main:

• AutoFormatTest#commentAfterRootElementIsRetained, #commentTrailingRootElementIsRetained
• RemoveContentTest#trailingCommentKeepsItsOwnLine, #precedingCommentOnTheLineOfAnEarlierSiblingIsRetained

New regression guards for the behavior that must not change:

• AutoFormatTest#trailingWhitespaceAfterRootElementIsRemoved
• RemoveContentTest#precedingCommentOnItsOwnLineIsRemoved

:rewrite-xml:test, :rewrite-maven:test and :rewrite-gradle:test are green.

Not covered here

• A comment trailing the prolog (<?xml version="1.0"?> <!--generated-->) is still moved onto its own line, because Keep XML comments trailing an element on the same line #8353's predicate bails when the parent is not an Xml.Tag. Consistent with that PR's stated scope, inconsistent as a rule; worth a separate look if anyone hits it.