feat: enhance multi-search-replace

refactor(tools): Make applyDiffTool backward compatible and fix errors

Makes `src/core/tools/applyDiffTool.ts` backward compatible by
allowing it to process both the new XML-based `args` parameter
and the legacy `path`, `diff`, and `start_line` parameters.

This change also resolves TypeScript errors that arose from
duplicated code during the initial refactoring for multi-diff
support.

Author: samhvw8