ci(doc-linter): distinguish prose-bearing directive fences from code blocks

The line-length skip was

    test_line_length = not (in_md_fence or in_rst_codeblock)

`in_md_fence` was True for every MyST/Markdown fence regardless of
content type. That includes admonition directives like `:::{note}`,
`:::{warning}`, `:::{tip}` whose content is normal prose, not
preformatted code. Long lines in admonitions were silently skipped,
contradicting the documented 80-char rule which exempts code blocks
only.

Track an `is_code` property on each fence-stack entry. A fence is
code-bearing when:
- info string is empty (plain ``` per CommonMark), OR
- info string doesn't start with `{` (bare language tag like
  `python`, `bash`, `yaml`), OR
- info string is `{<directive>}` and `<directive>` is in the
  CODE_BEARING_DIRECTIVES set (`code-block`, `code`, `sourcecode`,
  `cfgcmd`, `opcmd`, `cmdinclude`, `cmdincludemd`, `literalinclude`,
  `parsed-literal`, `raw`, `command-output`, `eval-rst`).

Anything else is prose-bearing (`{note}`, `{warning}`, `{tip}`,
`{deprecated}`, `{seealso}`, …) and its content gets line-length
checked.

`in_md_code_fence` checks the topmost stack entry — the innermost
fence wins, so a `{note}` containing an inner `{code-block}` lints
the outer prose lines and skips the inner code-block body. The
classic `is_suppression_marker()` call still uses `in_md_fence`
because suppression markers are about "any fence depth" not
"code-bearing depth".

`{eval-rst}` is kept in CODE_BEARING_DIRECTIVES to preserve current
behavior — its body is RST and any line-length on nested
`.. code-block::` is handled by the separate RST tracker. Tightening
eval-rst is a separate change if wanted.

Test coverage:
- `_fence_is_code` classifier: 16 cases (code-like vs prose-like)
  all pass.
- Integration: long line in `{note}` flagged ✓; long line in
  ```python``` not flagged ✓; long line in `{cfgcmd}` not flagged ✓;
  nested `{note}` > ```text``` — inner skipped ✓; nested `{note}` >
  prose — flagged ✓.

Sweep over current `docs/` tree: 28 new warnings surface across the
existing pages (long prose inside admonition directives that the
previous logic had been silently hiding). CI on PR scope is changed
files only, so the new findings appear only when contributors touch
those pages — they won't break this PR or future infra PRs.

Tracked as item 5 of the rolling-side cleanup backlog from PR #2014
/ #2019 / #2020 reviews.

🤖 Generated by [robots](https://vyos.io)

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