RFR: 8356411: Comment tree not reporting correct position for label

[Hannes Wallnöfer]

Please review a change to fix the reported position of the label in `@see` tags in Markdown doc comments. When `MarkdownTransformer` would process the label of a `@see` tag together with the leading reference, it would set the position of the label to the position immediately following the reference, losing the actual source position of the label. The solution is to process the label without the leading reference. 

I have looked for other standard tags that may be affected by similar problems, but the `@see` tag is the only tag that [supports multiple forms](https://docs.oracle.com/en/java/javase/24/docs/specs/javadoc/doc-comment-spec.html#see) where Markdown text is stored in a list together with non-Markdown arguments.

The two tests are slightly redundant, but they test slightly different aspects of the fix and they were easy to implement in their respective frameworks.

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Commit messages:
 - 8356411: Comment tree not reporting correct position for label

Changes: https://git.openjdk.org/jdk/pull/26842/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26842&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356411
  Stats: 59 lines in 4 files changed: 48 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/26842.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26842/head:pull/26842

PR: https://git.openjdk.org/jdk/pull/26842