Honor --allow-private-ip on thv registry login --registry

[reyortiz3]

Summary

thv config set-registry --allow-private-ip <url> lets an operator point the CLI at a registry behind a private/corp IP. The matching option is missing from thv registry login, and even via the Go API the boolean was hardcoded one layer down — so an operator combining registration + login in a single step (thv registry login --registry http://10.x.y.z/api --issuer ... --client-id ...) always hits the private-IP rejection regardless of intent.

The same gap applies to auth.Login when used programmatically: LoginOptions had no way to express "this registry is allowed on a private IP."

This PR closes the gap symmetrically:

• pkg/registry/auth/login.go — adds AllowPrivateIP bool to LoginOptions. ensureRegistryURL threads it into config.DetectRegistryType (so the API-vs-static probe can actually reach the registry) and into the SetRegistryAPI / SetRegistryURL calls (which were false literals).
• cmd/thv/app/registry_login.go — adds -p / --allow-private-ip, mirroring the flag on thv config set-registry. Default is false.
• docs/cli/thv_registry_login.md — regenerated via task docs.
• pkg/registry/auth/login_test.go — new table case in TestEnsureRegistryURL proving AllowPrivateIP: true propagates to SetRegistryURL. The existing false-cases stay untouched, so the no-flag path is regression-guarded.

No behavior change for callers that leave the new field at its zero value.

Fixes #5351

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)
• CLI docs regenerated (task docs)
• Manual: go build ./cmd/thv + thv registry login --help shows the new -p, --allow-private-ip flag.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.43%. Comparing base (2894982) to head (828ef4a).

Files with missing lines	Patch %	Lines
pkg/registry/auth/login.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5353      +/-   ##
==========================================
+ Coverage   68.38%   68.43%   +0.05%     
==========================================
  Files         624      624              
  Lines       63442    63442              
==========================================
+ Hits        43386    43419      +33     
+ Misses      16818    16784      -34     
- Partials     3238     3239       +1     

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