Fix multi-character newline handling in IndentedStringBuilder.TrimEnd

[Copilot]

The TrimEnd method assumed single-character newlines, breaking when NewLine is configured to multi-character sequences like \r\n.

Changes:

• FindLastNewLineIndex: Changed signature from char to string parameter; now searches for complete newline sequence instead of just the last character
• Offset calculations: Replace hardcoded + 1 with + NewLine.Length at lines 289 and 298 to properly skip/preserve the entire newline sequence

Example:

var builder = new IndentedStringBuilder { NewLine = "\r\n" };
builder.AppendLine("foo").TrimEnd();
// Previously: Would incorrectly calculate positions mid-sequence (\r|\n split)
// Now: Correctly handles the full \r\n sequence

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[CLAassistant]

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