fix: fail when discover of auth creds fail

[yrobla]

instead of falling back to unauthenticated, make the process fail when there is a problem in discovering auth config

[Copilot]

Pull request overview

This PR implements a critical security improvement by changing the authentication failure handling from "fail open" (falling back to unauthenticated access) to "fail closed" (refusing to create a backend). When an MCPServer references authentication configuration that fails to load (missing auth config or secrets), the system now returns nil instead of silently proceeding without authentication.

Key Changes:

• Modified mcpServerToBackend to return nil when auth discovery fails instead of logging a warning and continuing
• Added nil check in GetWorkloadAsVMCPBackend to handle auth discovery failures
• Updated error logging from Warnf to Errorf to reflect the severity of auth failures
• Updated tests to expect nil backend when auth config or secrets are missing

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/vmcp/workloads/k8s.go	Implements fail-closed behavior by returning nil from mcpServerToBackend when auth discovery fails, and adds nil check in GetWorkloadAsVMCPBackend
pkg/vmcp/workloads/k8s_test.go	Updates test expectations for TestDiscoverAuth_AuthConfigNotFound and TestDiscoverAuth_SecretNotFound to verify nil backend is returned

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.30%. Comparing base (66fff4a) to head (f339996).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2758      +/-   ##
==========================================
+ Coverage   56.28%   56.30%   +0.01%     
==========================================
  Files         319      319              
  Lines       30749    30753       +4     
==========================================
+ Hits        17308    17315       +7     
+ Misses      11953    11951       -2     
+ Partials     1488     1487       -1     

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