Add tests for coverage gaps

[snabb]

Summary

• add store coverage for memory, file, limited, and default store implementations
• add HTTPReaderAt tests for metadata accessors, malformed partial responses, validation failures, EOF behavior, and fallback store usage
• raise main package coverage from 39.6% to 90.6% while keeping the suite green

Testing

• go test ./... -coverprofile=coverage.out

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.18%. Comparing base (026354e) to head (1f2f99d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #6       +/-   ##
===========================================
+ Coverage   38.18%   83.18%   +45.00%     
===========================================
  Files           3        3               
  Lines         220      220               
===========================================
+ Hits           84      183       +99     
+ Misses        119       19      -100     
- Partials       17       18        +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.