Use markdown-it-py to robustly detect fenced code blocks and prevent placeholder desync

[skytin1004]

Purpose

Improve detection and handling of fenced code blocks in Markdown so that:

• Code chunks are never split during chunking.
• Placeholder insertion/restoration is stable and order-preserving.
• Single/unmatched fences (e.g., a lone ``` in text) do not corrupt placeholder order or chunk boundaries.

Description

This PR replaces regex-based code-block detection with a spec-compliant parser using markdown-it-py.

• Regex r"```[\s\S]*?```" previously used in replace_code_blocks() and indirectly in chunking fails with unmatched fences and various edge cases (e.g., ~~~, variable fence length, info strings).
• Using markdown-it-py yields consistent, spec-conformant behavior and fixes placeholder order issues and accidental code splits.

Before:

• A single unmatched ``` in the document could derail placeholder ordering and lead to broken chunking.

After:

• Only well-formed fenced blocks are replaced and treated as atomic during chunking.
• Unmatched/partial fences remain as plain text and no longer cause placeholder desynchronization.

Related Issue

• Fixes ``` not handled correctly (en→ja) #221 (``` not handled correctly)

Does this introduce a breaking change?

• Yes
• No

Notes:

• Behavior changes are strictly bug fixes and robustness improvements.
• Consumers should run dependency install to fetch markdown-it-py.

Type of change

• Bugfix
• Refactoring (no functional API changes)
• Feature
• Code style update
• Documentation content changes
• Other... Please describe:

Checklist

• I have thoroughly tested my changes
• All existing tests pass
• I have added new tests (if applicable)
• I have followed the Co-op Translator coding conventions
• I have documented my changes (if applicable)

Additional context

• The parser-based approach also unlocks future enhancements:
  • Language-aware handling via token.info (e.g., mode-specific rules for python, bash, etc.).
  • Optional handling for ~~~ fences and variable-length fences without extra complexity.
• Suggested follow-up: add targeted tests for:
  • Single unmatched fence,
  • Mixed ``` and ~~~ fences,
  • Variable fence lengths,
  • Very long code blocks to ensure atomic chunking.

[skytin1004]

I have reviewed the changes and everything looks good.