Add integration tests for CIMD embedded auth server support

[amirejaz]

Summary

• Stacked on Wire CIMD config through embedded AS and enable storage decorator #5348 (wire CIMD config through embedded AS). Merge that first.
• Adds four integration tests in test/integration/authserver/ that exercise the CIMD feature end-to-end against a real EmbeddedAuthServer instance — no mocks, no binary required:
  1. Discovery — client_id_metadata_document_supported: true appears in both /.well-known/oauth-authorization-server and /.well-known/openid-configuration when CIMD is enabled; absent when disabled
  2. Authorize accepts CIMD URL — /oauth/authorize with a CIMD URL as client_id redirects to the upstream IDP without prior DCR registration
  3. Authorize rejects CIMD URL when disabled — same request fails when CIMD is off
  4. No DCR required — flow completes without any call to /oauth/register, proving on-the-fly client resolution works

Also adds WithCIMD functional option to the integration test helper in test/integration/authserver/helpers/authserver.go.

Type of change

• Test coverage (no production code change)

Test plan

• go test ./test/integration/authserver/... passes
• task lint-fix clean

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.79%. Comparing base (90db2a6) to head (f9e761d).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           cimd-phase2-pr6-cimd-validation    #5379      +/-   ##
===================================================================
- Coverage                            68.79%   68.79%   -0.01%     
===================================================================
  Files                                  627      627              
  Lines                                63668    63672       +4     
===================================================================
  Hits                                 43801    43801              
- Misses                               16612    16614       +2     
- Partials                              3255     3257       +2     

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