Fixes #3106: Update model version check to only verify US versions

[anth-volk]

Summary

• Updates the model version alignment check script to use Modal gateway instead of GCP
• Removes UK version checking since policyengine-uk doesn't support the older Python versions that API v1 runs on
• Adds explanatory comments documenting why UK versions are not checked
• Maintains backwards compatibility by accepting but ignoring the -uk flag

Test plan

• Verified script works with US version check only
• Confirmed script correctly ignores UK version flag
• Tested failure case when US version doesn't exist

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.53%. Comparing base (af95d34) to head (647768b).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3107   +/-   ##
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  Coverage   70.53%   70.53%           
=======================================
  Files          55       55           
  Lines        2386     2386           
  Branches      339      339           
=======================================
  Hits         1683     1683           
  Misses        642      642           
  Partials       61       61           

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