test: add tests for escaped marker handling in multi-search-replace

Add tests that validate:
- Original content with unescaped markers
- Search content with escaped markers to match unescaped markers in original content
- Proper validation of escaped search, separator, and replace markers in diff content
- Successful application of diffs with escaped markers in search content

Signed-off-by: Eric Wheeler <roo-code@z.ewheeler.org>

Author: Eric Wheeler