fix: preserve trailing newlines in stripLineNumbers for apply_diff (#8020)

[roomote]

Description

This PR fixes issue #8020 where line numbers at the last line were not being stripped correctly when using apply_diff with SEARCH/REPLACE blocks.

Problem

The regex pattern in multi-search-replace.ts was capturing content with optional trailing newlines using (?:\n)?, which removed the trailing newline before passing content to stripLineNumbers(). This caused the stripLineNumbers function to fail on the last line because it couldn't match the line number pattern without a proper line ending.

Solution

Modified the stripLineNumbers function in src/integrations/misc/extract-text.ts to preserve the original trailing newlines. The function now:

1. Processes each line to strip line numbers
2. Preserves the original line ending style (CRLF or LF)
3. Maintains trailing newlines if they existed in the original content

Testing

• Added comprehensive test cases to verify the fix works for:
  • Simple cases with line numbers on the last line
  • Windows CRLF line endings
  • Complex multi-line scenarios from the original issue
• All existing tests pass (60/60 in multi-search-replace.spec.ts)
• No regression in functionality

Related Issue

Fixes #8020

Checklist

• Tests added/updated
• All tests passing
• No breaking changes
• Code follows project conventions

---

Important

Fixes trailing newline preservation in stripLineNumbers in extract-text.ts and adds comprehensive tests.

• Behavior:
  • Fixes stripLineNumbers in extract-text.ts to preserve trailing newlines.
  • Handles both CRLF and LF line endings.
• Testing:
  • Adds tests in multi-search-replace-issue-8020.spec.ts, multi-search-replace-line-number.spec.ts, and multi-search-replace-simple.spec.ts.
  • Tests cover simple cases, Windows CRLF endings, and complex multi-line scenarios.
• Misc:
  • Fixes issue [BUG] apply_diff fail to strip the line number at last line in SEARCH and REPLACE #8020.

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[roomote]

Reviewing my own code is like debugging in a mirror - everything looks backward but the bugs are still mine.

[roomote]

Is this approach intentional? The function detects line endings by checking if ANY '\r\n' exists in the content. This could cause issues with mixed line endings - for example, if a file has mostly LF endings with just one CRLF, it would treat everything as CRLF. Would it be better to check which line ending is more prevalent, or handle mixed endings differently?

[roomote]

Nice fix for preserving trailing newlines! Consider enhancing this comment to explicitly mention that this addresses the issue where the regex in multi-search-replace.ts removes trailing newlines with (?:\n)?. This would help future maintainers understand the context better.

[roomote]

Good test coverage for CRLF line endings! Could we also add a test case with mixed line endings (both CRLF and LF in the same content) to ensure the function handles this edge case correctly? This would catch the potential issue I mentioned about line ending detection.

[daniel-lxs]

The root cause remains in the parser regex in
src/core/diff/strategies/multi-search-replace.ts.

The pattern still optionally consumes the trailing newline for the SEARCH/REPLACE captures (via the optional (?:\n)? near the block boundaries), which drops information and forces stripLineNumbers() to reconstruct terminal newlines. The parser should avoid discarding data.

Action: adjust the regex so the trailing newline is not consumed (keep it outside the capture via lookahead, or explicitly capture and re-emit), especially around the ======= and >>>>>> REPLACE boundaries.

For example, change the segment that ends the SEARCH capture from:

([\s\S]?)(?:\n)?(?:(?<=\n)(?<!)=======\s\n)

to:

([\s\S]?)(?=(?<=\n)(?<!)=======\s\n)

and do the analogous change for the REPLACE block.

This keeps stripLineNumbers focused on stripping prefixes without having to preserve/re-add terminal newline semantics.