refine preprocess error reporting

[vbanos]

Stop using panic() inside the preprocess(), return the error in the main preprocessor().

This is relevant to #473

[codecov-commenter]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.41%. Comparing base (032cbd6) to head (03d3b7b).
⚠️ Report is 69 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/pkg/preprocessor/preprocessor.go	16.66%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #474      +/-   ##
==========================================
- Coverage   56.45%   56.41%   -0.05%     
==========================================
  Files         130      130              
  Lines        8052     8056       +4     
==========================================
- Hits         4546     4545       -1     
- Misses       3145     3149       +4     
- Partials      361      362       +1     

Flag	Coverage Δ
e2etests	40.63% <16.66%> (-0.04%)	⬇️
unittests	29.46% <0.00%> (-0.02%)	⬇️

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[Copilot]

Pull Request Overview

This PR refines error handling in the preprocessor by converting panic calls to proper error returns within the preprocess() function. The main change replaces several panic() calls with return err statements and adds error handling in the calling function.

• Converts preprocess() from a void function to one that returns an error
• Replaces internal panic() calls with proper error returns
• Adds error handling in the worker() method that still panics but with better error messaging

[NGTmeaty]

Looks good! Thank you!