Return error when custom registry is unreachable

[JAORMX]

Summary

• Return an error when a custom registry is configured but unreachable, instead of silently falling back to the default registry
• Add validation at creation time for RemoteRegistryProvider to ensure the registry is reachable
• Update tests to verify the new error behavior

Fixes #2764

Test plan

• Unit tests pass (task test)
• Linting passes (task lint)
• Manual testing: Set unreachable custom registry with thv config set-registry https://non-existent.example.com/registry.json and verify thv registry list and thv run <server> return errors

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.58%. Comparing base (15f0beb) to head (d40aa33).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/registry/factory.go	70.00%	3 Missing ⚠️
pkg/registry/provider_remote.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2879      +/-   ##
==========================================
- Coverage   56.59%   56.58%   -0.02%     
==========================================
  Files         322      322              
  Lines       31439    31444       +5     
==========================================
- Hits        17794    17793       -1     
- Misses      12110    12121      +11     
+ Partials     1535     1530       -5     

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