Encourage use of start_line in multi-file diff to match legacy diff

[mrubens]

To match

Roo-Code/src/core/diff/strategies/multi-search-replace.ts

Line 109 in c10fbbc

:start_line: (required) The line number of original content where the search block starts.

Closes #4744

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Important

Make start_line parameter mandatory in diff operations to match legacy behavior.

• Behavior:
  • Change start_line from optional to required in multi-file-search-replace.ts and multiApplyDiffTool.ts.
  • Affects diff processing and validation, ensuring start_line is always specified.
• Documentation:
  • Update comments and error messages in multi-file-search-replace.ts to reflect start_line requirement.
  • Modify XML parsing error message in multiApplyDiffTool.ts to indicate start_line is required.

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[daniel-lxs]

LGTM