fix: return lexer errors only if they affect the current cursor position

[luantranminh]

Currently, when doing completion in a file that has lexer errors (like unclosed quotes), completion is not working perfectly when applying. This creates a poor user experience as users often need completion while typing incomplete code.

Example:

resource "a" {
  one {
    sr  # User wants completion here
    two {
      name = "ok  # Unclosed quote
    }
  }
}

After selecting completion item, text will append instead of replacing, causing duplication:

resource "a"  {
  one {
    ssrc  # 'src' was appended to 'sr' instead of replacing it
    two {
      name = "ok  
    }
  }
}

Expected behavior:

• Completion should correctly replace existing text rather than appending
• Only append completion if the lexer error affects the current cursor position