Fix byte/character offset confusion in formatter for multi-byte UTF-8

[Copilot]

Formatting proto files with multi-byte UTF-8 characters (Cyrillic, etc.) was non-idempotent, adding empty // comment lines on each format operation.

Root Cause

offset_to_position in src/formatter/clang.rs converted clang-format's byte offsets to LSP positions using byte arithmetic:

let character = offset - last_newline;  // treats byte offset as character offset

This fails for multi-byte UTF-8. Example: byte offset 134 in Cyrillic text → calculated position 119 → should be 77 UTF-16 code units.

Changes

• Fixed offset calculation: Count UTF-16 code units from last newline instead of byte arithmetic

  let text_after_newline = &up_to_offset[last_newline..];
  let character = text_after_newline.encode_utf16().count();

• Added tests: test_offset_to_position_cyrillic (unit) and test_textedit_from_clang_output_cyrillic (integration) with multi-byte UTF-8 input

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Formatting inserts a new empty // comment line every time (non-idempotent formatting)</issue_title>
<issue_description>Hi! I just so happen to stumble upon this tricky bug in formatting.

What happened

I have this example:

message Test {
  // Обратная совместимость: если true, применяет фильтры enabled_not_false и removed_not_true.
  int32 x = 1;
}

When applying a formatting it seem to try to split the comment in two and spread among two lines. But in the end it just adds a new line with empty comment:

message Test {
  // Обратная совместимость: если true, применяет фильтры enabled_not_false и removed_not_true.
  // 
  int32 x = 1;
}

Further formatting just add empty line comments.

message Test {
  // Обратная совместимость: если true, применяет фильтры enabled_not_false и removed_not_true.
  // 
  // 
  // 
  int32 x = 1;
}

Environment

• OS: Linux fedora 42
• Neovim: NVIM v0.11.1
• protols: 0.13.2
• clang-format: clang-format version 20.1.8 (Fedora 20.1.8-4.fc42)
• Formatting trigger: Neovim LSP (vim.lsp.buf.format()), also happens on :w (format-on-save enabled)

Video Example

https://github.com/user-attachments/assets/f33be03e-78e6-45db-8c83-89ae15a31d0b
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Formatting inserts a new empty // comment line every time (non-idempotent formatting) #104

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