Show issue hovers in Markdown/plaintext and in YAML strings - #9147

Issue hovers don't appear in Markdown or YAML files. Outside Markdown and plaintext, shouldShowHover only allows hovers on comment tokens.

• Markdown/plaintext: Move markdown/plaintext check into completion providers #8049 moved the Markdown/plaintext exception out of isComment and into the completion providers only. shouldShowHover still calls isComment, so issue and user hovers stopped working in prose (those tokens are Other).
• YAML: comments already work. Issue references usually sit in string values, though, and the YAML grammar tokenizes those as String:

changes:
  - Fixed crash (#123)        # `#` not preceded by whitespace → plain scalar
  - "Quoted fix for #123"
notes: |
  Block scalar mentioning #123

Changes

• shouldShowHover (src/issues/util.ts)
  • Returns true anywhere in markdown/plaintext, as before Move markdown/plaintext check into completion providers #8049.
  • Otherwise returns true for comment tokens, and for string tokens when the document language is in a new optional stringLanguageIds parameter.
  • Calls getTokenInformationAtPosition directly instead of going through isComment. isComment is unchanged for the completion and TODO providers.
• IssueHoverProvider: passes ['yaml'].
• UserHoverProvider: unchanged. YAML strings often contain @ that isn't a user (actions/checkout@v4, emails, @types/node), and each hover would trigger a user lookup.
• Tests: shouldShowHover cases in src/test/issues/issuesUtils.test.ts, with vscode.languages.getTokenInformationAtPosition stubbed so they don't depend on grammar loading.

Review notes

• In YAML, the issue hover now also matches digit-only hex strings like '#123456'. Unqualified references are still capped at the repo's max issue number, so these only produce a hover if that issue exists.
• Only the yaml language ID is opted in. dockercompose and github-actions-workflow (GitHub Actions extension) stay comment-only.

• Fixes Issue hover not working for YAML and Markdown files #9125