refactor(convert): restructure Nydus-to-OCI conversion workflow

[J-jxr]

Overview

This pull request refactors the Nydus-to-OCI reverse conversion workflow (originally implemented in PR #1741) by introducing explicit usage of acceleration-service and nydus-snapshotter APIs.

Related Issues

Related #1741, #1682

Change Details

• Integrated acceleration-service APIs: Replaced custom conversion logic with calls to acceleration-service interfaces .
• Adopted nydus-snapshotter APIs: Utilized snapshotter-specific methods for image layer validation and format conversion.
• Removed obsolete custom logic: Eliminated handcrafted layer parsing and format conversion code that was no longer needed after adopting the third-party APIs.

Test Results

• Validated end-to-end Nydus-to-OCI conversion functionality with the refactored logic, confirming output consistency with the original implementation.

Change Type

Code Refactoring

Self-Checklist

• I have run a code style check and addressed any warnings/errors
• I have added appropriate comments to my code (if applicable).

[codecov]

Codecov Report

❌ Patch coverage is 22.91667% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.05%. Comparing base (340b2cb) to head (2198f0a).

Files with missing lines	Patch %	Lines
...ontrib/nydusify/pkg/converter/reverse_converter.go	24.44%	34 Missing ⚠️
contrib/nydusify/cmd/nydusify.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1781      +/-   ##
==========================================
+ Coverage   54.87%   55.05%   +0.18%     
==========================================
  Files         199      199              
  Lines       54111    53709     -402     
  Branches    44727    44727              
==========================================
- Hits        29693    29571     -122     
+ Misses      22930    22660     -270     
+ Partials     1488     1478      -10     

Files with missing lines	Coverage Δ
contrib/nydusify/cmd/nydusify.go	16.14% <0.00%> (+0.38%)	⬆️
...ontrib/nydusify/pkg/converter/reverse_converter.go	35.63% <24.44%> (+1.30%)	⬆️

... and 3 files with indirect coverage changes

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[BraveY]

Just delete the unused variable.

[J-jxr]

@BraveY, thanks for the review. I kept this execCommand variable intentionally as it's reserved for unit tests later - I'll use it to mock exec.Command in test cases. Will remove it if it turns out unnecessary eventually.