Clear all cached registry tokens on logout

[rdimitrov]

Summary

thv registry logout left the cached access token stored under <key>_AT behind, so the next thv registry login short-circuited through tier 2 of the shared OAuthTokenSource and returned the cached token without ever triggering a browser flow. The same gap also left stale entries under derived keys after a registry URL or issuer change.

• pkg/registry/auth/login.go — Logout now lists every secret visible to the registry-scoped provider and deletes them in one shot, mirroring pkg/llm.DeleteCachedTokens. The refresh-token key, its _AT companion, and any stale entries from previous configs are all cleared in a single pass.
• pkg/registry/auth/login_test.go — replaces the targeted-delete tests with cases covering: full cleanup (including _AT and a stale entry), no-op on empty list, no-op for providers that can't list/delete, error propagation on ListSecrets/DeleteSecrets, and config-update error propagation.

Closes #5373

Type of change

• Bug fix (non-breaking change which fixes an issue)

Test plan

• Unit tests added/updated — go test -race ./pkg/registry/auth/... ./pkg/auth/tokensource/... ./pkg/llm/... passes
• task lint-fix reports 0 issues

Does this introduce a user-facing change?

thv registry logout now fully clears all cached registry OAuth credentials, so a subsequent thv registry login always triggers a fresh browser auth flow rather than silently reusing a cached access token.

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.81%. Comparing base (2043cf6) to head (56d311d).

Files with missing lines	Patch %	Lines
pkg/registry/auth/login.go	93.75%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5382   +/-   ##
=======================================
  Coverage   68.81%   68.81%           
=======================================
  Files         627      627           
  Lines       63594    63600    +6     
=======================================
+ Hits        43762    43767    +5     
+ Misses      16583    16581    -2     
- Partials     3249     3252    +3     

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