Added new apis for CutterDiff: rz_unified_diff_grouped and rz_diff_op_stringify

[NewtronReal]

cleanup

Your checklist for this pull request

• I've read the guidelines for contributing to this repository.
• I made sure to follow the project's coding style.
• I've documented every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
• I've updated the Rizin book with the relevant information (if needed).
• I've used AI tools to generate fully or partially these code changes and I'm sure the changes are not copyrighted by somebody else.

Detailed description

Added two new apis for passing RzDiffOp groups(with binarydiffing selector) to cutter and stringifying specific RzDiffOp independently. Seperated fill char bounds from the main function.

Added necessary test cases.
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Test plan
Passes test_rz_diff_op_stringify in tests.

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Closing issues

None

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[codecov]

Codecov Report

❌ Patch coverage is 80.30303% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.96%. Comparing base (d2d6846) to head (76f4a38).

Files with missing lines	Patch %	Lines
librz/diff/unified_diff.c	79.68%	4 Missing and 9 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
librz/cons/pal.c	65.26% <100.00%> (+0.18%)	⬆️
librz/core/cmd_descs/cmd_descs.c	50.63% <ø> (ø)
librz/diff/unified_diff.c	79.37% <79.68%> (-0.32%)	⬇️

... and 10 files with indirect coverage changes

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[wargio]

you should maybe change the original code to use this function too. after all it's an extract