Do not rely on Docker-Content-Digest when getting manifests

[jonesbusy]

Description

Fix #522

Testing done

mvn clean install and enable again test

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.60%. Comparing base (f571253) to head (e7c6b92).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/main/java/land/oras/Registry.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #526      +/-   ##
============================================
+ Coverage     87.85%   88.60%   +0.74%     
+ Complexity      663      662       -1     
============================================
  Files            39       39              
  Lines          2018     2027       +9     
  Branches        231      233       +2     
============================================
+ Hits           1773     1796      +23     
+ Misses          156      140      -16     
- Partials         89       91       +2     

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