ci: stack-based fence tracking + file-ext-aware suppression markers

Two issues from PR review:

1. MD/MyST fence tracking treated any longer same-char fence as a
   closer, which would close `:::{note}` (3 cols) when seeing a
   nested `::::{code-block}` (4 cols) opener inside it. Real bug
   in `docs/configuration/interfaces/wireless.md:198–209` (currently
   unobservable because inner code lines are <80 chars).

   The "opener has info string / closer has none" heuristic is not
   sufficient on its own: there are 2,826 bare-fence opens in the
   tree, so info-string presence cannot distinguish opener from
   closer.

   Fix: stack-based tracking. A fence is treated as a closer only
   when (a) the stack is non-empty, (b) char and length match the
   top, AND (c) no info string follows. Anything else opens a new
   (possibly nested) fence. The outermost fence's info string still
   determines the `md_fence_is_eval_rst` flag.

2. `is_suppression_marker()` accepted `% stop_vyoslinter` in any
   file outside an MD fence. Per AGENTS.md and the doc-linter
   instructions, MyST `% ...` markers are only valid in `.md`
   files; a stray `% stop_vyoslinter` in `.rst`/`.txt` should not
   silently disable linting. Pass `file_ext` and gate the marker
   forms accordingly: `% ...` only in `.md` outside fences;
   `.. ...` in `.rst`/`.txt` outside RST code-blocks, or in `.md`
   inside an `{eval-rst}` fence.

3. Drop the `not in_rst_codeblock` guard on `.. code-block::`
   detection. Each occurrence resets the tracked indent (matches
   `origin/rolling` baseline). Without this, code-block-inside-
   code-block kept the outer indent and broke dedent detection
   (verified regression: `_rst_legacy/configuration/system/
   rst-syslog.rst:216` long-line warning was lost; restored).

Verified:
- All 7 original synthetic fixtures pass.
- New fixture `nested.md` (3-col outer wraps 4-col inner with long
  line in between fences) produces exactly one warning at the line
  outside both fences.
- New fixture `wrongmarker.rst` (`%` in `.rst`) — IP error fires
  (marker correctly ignored).
- Full-tree run vs origin/rolling baseline: zero regressions on
  pre-existing `.rst`/`.txt` warnings; all new output is `.md`.

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

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