Fix mvnup comment formatting for override comments

[gnodet]

Summary

• Fix PluginUpgradeStrategy to set precedingWhitespace on Comment nodes inserted before plugin elements, so the "Override version inherited from parent" comment appears on its own line instead of being appended to the preceding </plugin> closing tag
• Add test assertions to verify comments are not on the same line as closing tags

Problem

When mvnup adds plugin management entries for plugins inherited from a remote parent, it inserts a <!-- Override version inherited from parent --> comment before the <plugin> element. The Comment.of(...) factory creates a comment with no precedingWhitespace, causing it to render on the same line as the previous </plugin> closing tag:

            </plugin><!-- Override version inherited from parent -->
            <plugin>

This breaks projects that enforce POM formatting (e.g. via spotless-maven-plugin), which expect:

            </plugin>
            <!-- Override version inherited from parent -->
            <plugin>

Fix

Set precedingWhitespace on the Comment node to match the plugin element's indentation at both insertion sites in PluginUpgradeStrategy:

• addPluginManagementEntryFromUpgrade() — for build/pluginManagement/plugins
• addDirectPluginOverrides() — for build/plugins

Fixes gnodet/maven4-testing#23134

Test plan

• Existing PluginUpgradeStrategyTest passes (31 tests)
• New assertions verify comments are on their own line (assertFalse(xml.contains("</plugin><!--"), ...))
• Full maven-cli module verify passes (468 tests)

🤖 Generated with Claude Code

[gnodet]

Note: This PR is a workaround for a missing API in DomTrip. The Editor class handles whitespace automatically for element insertion (insertElementBefore/After) and comment appending (addComment), but has no positional comment insertion method (insertCommentBefore/After). This forced mvnup to use the raw ContainerNode.insertChildBefore API which doesn't handle precedingWhitespace.

Filed upstream: maveniverse/domtrip#254

Once DomTrip adds Editor.insertCommentBefore(), the manual precedingWhitespace() calls here should be replaced with the Editor API.

[gnodet]

Updated: upgraded DomTrip to 1.6.0 which includes the new Editor.insertCommentBefore() / insertCommentAfter() API (maveniverse/domtrip#254). The manual precedingWhitespace() workaround has been replaced with the proper Editor API call.