fix: Improve case-sensitive handling

[DaveTryon]

As called out in #523, we have some issues if a manifest file contains files whose names vary only by case. When this happens, we create a NullReferenceException that causes the validator to terminate prematurely without providing information to allow the person to address the issue. This PR does the following:

It leaves the overall validation logic unchanged, but it detects the case that was causing the NullReferenceException and reports it as an AdditionalFile error. This is consistent with the effect of copying 2 files whose names vary only by case to a file case-insensitive drive. This change allows the validation to complete with errors, rather than exiting prematurely

If errors are found when scanning on a case-insensitive system, it adds a brief warning to inform the users that if they created the manifest on a case-sensitive system, they may need to validate it on a case-sensitive system. We already have a mechanism to identify case-sensitivity, but it was private inside a class. It is now exposed on the associated interface. Unit tests have been updated to account for the case sensitivity check.

I simulated this case locally by generating a manifest, then manually creating some cases where file entries differed by case and by hash. When the validator runs, the order in which the manifest entries is processed is nondeterministic. As a result, the same manifest file can produce different errors. Depending on the order of processing, the errors may be reported as invalid hashes or as additional files. Because of this, I made the case sensitivity warning general and displayed it at the top of the output results. If any errors are found when validation is performed on a case-insensitive OS, the error header now reads as follows:

------------------------------------------------------------
Individual file validation results
  Note: If the manifest file was originally created using a
        case-sensitive OS, you may also need to validate it
        using a case-sensitive OS.
------------------------------------------------------------

There was also a private member in the OSUtils class that was not readonly and the compiler complained about it. I fixed it while I was touching the file.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.50000% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 59.60%. Comparing base (44dc753) to head (887d2e1).

Files	Patch %	Lines
...osoft.Sbom.Api/Workflows/Helpers/FilesValidator.cs	0.00%	7 Missing and 1 partial ⚠️
...Api/Workflows/SBOMParserBasedValidationWorkflow.cs	75.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #550      +/-   ##
==========================================
- Coverage   59.66%   59.60%   -0.06%     
==========================================
  Files         248      248              
  Lines        7581     7595      +14     
  Branches      902      905       +3     
==========================================
+ Hits         4523     4527       +4     
- Misses       2645     2652       +7     
- Partials      413      416       +3     

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