.Net: Fix TextChunker.SplitPlainTextLines to actually split on newlines regardless of token count

[shethaadit]

Description

Summary

Fixes issue #12556 where TextChunker.SplitPlainTextLines does not actually split text on newlines when the input token count is less than the maxTokensPerLine parameter.

Problem

The SplitPlainTextLines method had two issues:

1. Incorrect separator: Used "\n\r" which looks for text containing both newline AND carriage return, rather than splitting ON newlines
2. Token-first logic: Only attempted splitting when token count exceeded the limit, ignoring newlines in shorter texts

This caused the method to return unsplit text with preserved newline characters instead of separate lines, which was counterintuitive given the method name.

Solution

• Added a dedicated s_plaintextLineSplitOptions array with "\n" as the first separator specifically for line splitting
• Modified SplitPlainTextLines to always split on newlines first, regardless of token count
• Applied token-based splitting to individual lines only if they exceed the token limit
• Preserved existing behavior for SplitPlainTextParagraphs by keeping the original split options

Changes

• Added: New s_plaintextLineSplitOptions array for proper line splitting behavior
• Modified: SplitPlainTextLines method to prioritize newline splitting over token limits
• Preserved: Existing SplitPlainTextParagraphs functionality unchanged

Testing

• ✅ Fixes the failing test case from issue .Net: Bug: TextChunker does not actually split on newlines #12556
• ✅ All existing tests continue to pass, including CanSplitTextParagraphsOnNewlines
• ✅ Maintains backward compatibility for paragraph splitting behavior

Verification

The method now correctly splits this input:

"First line\nSecond line\nThird line"

Fixes #12557