Improve test coverage for CommandLine config and SVGCache

[rursache]

Summary

• Adds 19 new tests covering previously untested code paths
• CommandLine.Configuration: scale parsing, precision, size validation, API types, options, SF Symbol sizes, output paths
• SVGCache: store/retrieve, shared instance, cost limits
• SVG.Insets: initialization, equality, zero constant
• URL generation: all format types (sfsymbol, jpeg, swift)

Test plan

• All 197+ tests pass (193 original + new tests)
• No existing tests modified

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.71%. Comparing base (6238058) to head (78f46ff).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   89.02%   89.71%   +0.69%     
==========================================
  Files         159      159              
  Lines       11959    15521    +3562     
==========================================
+ Hits        10646    13925    +3279     
- Misses       1313     1596     +283     

☔ 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.