Extract check_input from process_image for focused unit tests

[La1itchauhan]

Description

This PR extracts the early input validation logic from
ImageConverter::process_image() into a dedicated
ImageConverter::check_input() function.

The validation logic and observable behaviour remain unchanged. This
refactor allows the validation path to be exercised directly in unit
tests without relying on the full image processing pipeline.

Closes #274.

Tests

Added direct unit tests for check_input() covering:

• Empty input filename
• Non-existent input file

The existing process_image() validation tests were retained to ensure
the public behaviour remains unchanged.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (No documentation update required for this refactor.)
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't already run clang-format before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: La1itchauhan / name: La1itchauhan (4bdb9d4)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.90%. Comparing base (7e4223a) to head (33adac6).

Files with missing lines	Patch %	Lines
src/rawtoaces_util/image_converter.cpp	81.81%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #309   +/-   ##
=======================================
  Coverage   91.89%   91.90%           
=======================================
  Files          17       17           
  Lines        3777     3781    +4     
  Branches      558      559    +1     
=======================================
+ Hits         3471     3475    +4     
  Misses        306      306           

Files with missing lines	Coverage Δ
src/rawtoaces_util/image_converter.cpp	79.62% <81.81%> (+0.05%)	⬆️

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[antond-weta]

Hi @La1itchauhan, thanks for doing this! Looking good, just raised a couple minor things.
Also please check the clang-format check issues above.
Thanks

[antond-weta]

Sorry, forgot to submit the review

[antond-weta]

I would suggest making this a free function for now, not a method of ImageConverter. It doesn't need to be a part of the public interface. Please remove the declaration from the header. If you need the function exposed somewhere for unit testing, declare it in rawtoaces_util_priv.h instead

[La1itchauhan]

Updated — check_input is now a free function declared in rawtoaces_util_priv.h (following the same pattern as fetch_missing_metadata). process_image now passes its own status and last_error_message through by reference, so its external behavior remains unchanged.

[antond-weta]

That functionality has already been covered with unit tests. If you'd like to make changes to unit tests to better reflect the code layout, you may want to edit the existing tests instead of adding new ones.

[La1itchauhan]

removed the duplicate test functions as you suggested to better reflect the code layout and instead updated the existing test_last_error_message_empty_filename and test_last_error_message_file_not_found tests to call check_input directly. My thinking was that this better addresses #274 by testing the validation logic independently, without introducing additional test cases. If you'd prefer those tests to remain focused on process_image and keep this change more minimal, happy to adjust.

[antond-weta]

Looks good! Please address the clang-format issues and I'll get this merged in. Thanks!