## Why
Updating a file with `apply_patch` historically normalized its contents to LF,
which can rewrite line endings outside the requested change.
## What changed
- Add an opt-in `PreserveLineEndings` update mode and thread it through patch
verification, application, and the standalone executable. Existing APIs keep
the LF-normalizing mode by default.
- Retain the original endings of untouched and context lines, including in
mixed-ending files, and use the file's first line ending for inserted or
replaced lines.
- Track patch context separately from identical changed lines so preservation
does not alter replacement ordering or matching.
## Testing
Add coverage for CRLF, CR, mixed endings, trailing blank lines, repeated lines,
EOF overlap rejection, and legacy LF normalization.
GitOrigin-RevId: 085f0cbc57e0965aa62c6a2d0e80870424e762f5
## Summary
This PR introduces an End to End test suite for apply-patch, so we can
easily validate behavior against other implementations as well.
## Testing
- [x] These are tests
Charlie Marsh