fix(authz): support namespaces filtering

[erka]

This PR enables the ListNamespaces API call, which is critical for the UI functionality. By default, the API will return all namespaces in the response. To ensure the response includes only the namespaces a user is permitted to access, the policy should specify a viewable_namespaces array containing the appropriate namespace strings.

closes #3688

[codecov]

Codecov Report

Attention: Patch coverage is 84.33735% with 13 lines in your changes missing coverage. Please review.

Project coverage is 64.63%. Comparing base (866ba43) to head (a237959).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/server/authz/engine/rego/engine.go	72.97%	7 Missing and 3 partials ⚠️
internal/server/authz/engine/bundle/engine.go	82.35%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3692      +/-   ##
==========================================
+ Coverage   64.55%   64.63%   +0.08%     
==========================================
  Files         170      170              
  Lines       17163    17233      +70     
==========================================
+ Hits        11079    11139      +60     
- Misses       5393     5399       +6     
- Partials      691      695       +4     

Flag	Coverage Δ
unittests	64.63% <84.33%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[markphelps]

ty!