[joselynrianaaa]

Fixes #6238

InvalidSnippet reported "This @snippet has no body" for external snippets that
reference their content with the class attribute, for example:

{@snippet class=com.example.Snippets region=builder lang=java}

Per the JavaDoc specification, the colon, newline and body may be omitted in an
external snippet, which is one that uses either file or class:
https://docs.oracle.com/en/java/javase/25/docs/specs/javadoc/doc-comment-spec.html#external-snippets

The check already exempted snippets with a file attribute but not class. This
change exempts both. A snippet that has no body and neither attribute is still
reported.

Tests added to InvalidSnippetTest:

• snippetWithoutBody_butWithClass: the case from the issue.
• snippetWithoutBody_butWithClassAndTrailingSpace: {@snippet class=HelloWorld },
  the form used in the JDK programmer's guide.
• snippetWithoutBody_andWithoutFileOrClass: confirms the warning still fires when
  there is no body and no external reference.

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