fix: treat closing fence longer than opener as code-block boundary in MarkdownHeaderSplitter (#12954)

[Taranum01]

Summary

Fixes #12954.

MarkdownHeaderSplitter previously matched any ``` (or longer) sequence as a code-fence boundary using a single regex. That made a 4-backtick opener close on the first 3-backtick run, and any bare ``` outside an open fence was treated as a stray boundary that swallowed subsequent headings.

Per CommonMark, a closing fence must use the same character as the opener and be at least as long. This PR replaces the single regex with a per-line scan that enforces both rules.

Changes

• haystack/components/preprocessors/markdown_header_splitter.py
  • Removed module-level _code_block_pattern (no longer used).
  • New per-line _is_closing_fence helper: same character as opener, length >= opener length, only whitespace follows.
  • _split_into_code_blocks walks the source line-by-line, opening/closing a block only when the line qualifies as a fence, and emits the original code_block lines verbatim.

Tests

• test/components/preprocessors/test_markdown_header_splitter.py
  • test_opening_fence_does_not_close_on_short_run: opener of length 4 stays open until a >=4 run is seen.
  • test_stray_short_fence_outside_code_block_does_not_swallow_headings: a bare 3-backtick line with no opener does not consume the following heading.

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