fix: handle non-BMP UTF-16 characters in markdown formatting

[Arondy]

Описание

Исправляет некорректное форматирование Markdown-ссылок при наличии в тексте non-BMP символов, например эмодзи.

Тип изменений

• Исправление бага
• Новая функциональность
• Улучшение документации
• Рефакторинг

Связанные задачи / Issue

Closes #62.

Тестирование

from pymax.formatting.markdown import Formatter

clean, entities = Formatter.format_markdown("🔥 [a](https://x.com) 👍 [b](https://y.com)")
assert entities[0].from_ == 3   # 🔥(2) + пробел(1)
assert entities[1].from_ == 8   # 🔥(2) + пробел(1) + a(1) + пробел(1) + 👍(2) + пробел(1)

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Improved handling of emoji and extended Unicode characters in markdown formatting.

[coderabbitai]

📝 Walkthrough

Walkthrough

The formatter now correctly handles non-BMP characters by tracking markdown positions in UTF-16 code units. A new infrastructure layer (BMP_MAX constant and _code_units_len helper) enables the parser to distinguish BMP characters (1 code unit) from non-BMP characters (2 code units), allowing accurate position advancement across LINK, HEADING, QUOTE, and character parsing paths.

Changes

UTF-16 Position Tracking

Layer / File(s)	Summary
UTF-16 Infrastructure src/pymax/formatting/markdown.py	Formatter gains BMP_MAX = 0xFFFF constant and _code_units_len() helper to compute UTF-16 code-unit lengths by encoding text as UTF-16LE and dividing by 2.
UTF-16 Position Tracking in Parsing src/pymax/formatting/markdown.py	LINK label extraction, HEADING/QUOTE text loops, and default character advancement now increment clean_pos using UTF-16 code-unit sizing (2 for non-BMP, 1 for BMP) instead of fixed +1 increments.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related issues

• #62: The UTF-16 code-unit position tracking changes directly address the bug where non-BMP characters shifted LINK positions by fixing offset calculations throughout the markdown formatter.

Poem

🐰 In UTF-16's binary dance,
Where code units bloom and prance,
BMP bounds are now our guide,
Non-BMP marked with widened stride,
Positions tracked both true and bright! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing handling of non-BMP UTF-16 characters in markdown formatting, which matches the core objective of the PR.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description follows the required template with all sections completed: description, change type, related issues, and testing example.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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