Added comprehensive unit tests for authorization logic:

- hasAccessToNamespace() tests:
  - User subject access (positive and negative cases)
  - Group subject access
  - ServiceAccount subject access
  - ServiceAccount with empty namespace (defaults to RoleBinding ns)
  - Privileged groups (system:masters, cozystack-cluster-admin)

- Get() handler tests:
  - Returns namespace when user has access
  - Returns NotFound when user lacks access (not Forbidden)
  - Returns NotFound for non-tenant namespaces

All tests verify that authorization correctly enforces RoleBinding-based
access control and prevents IDOR vulnerabilities.

Signed-off-by: IvanHunters <[email protected]>
(cherry picked from commit 5b2501d)

Address review feedback from sircthulhu and CodeRabbit:

1. Return Forbidden instead of NotFound for unauthorized access
   - Get() now returns 403 Forbidden to follow standard K8s RBAC behavior
   - Previously returned 404 NotFound for security-by-obscurity
   - Updated test expectations to match new behavior

2. Propagate field and label selectors in Watch handler
   - Pass opts.FieldSelector and opts.LabelSelector to upstream Watch
   - Add defensive filtering before authorization to prevent RBAC bypass
   - Fixes potential issue with resourceNames restrictions

3. Refactor subject-matching logic to eliminate duplication
   - Extract matchesSubject() helper for Group/User/ServiceAccount checks
   - Remove duplicated code from filterAccessible and hasAccessToNamespace
   - Consolidates ServiceAccount namespace fallback logic

All tests pass successfully.

Signed-off-by: IvanHunters <[email protected]>
(cherry picked from commit 7257b6a)

- Hoist user identity extraction out of the Watch goroutine; reuse a
  cached username and groups map across events instead of re-fetching
  them per event. Watch now returns Unauthorized up front when no user
  is present in the context, rather than failing silently per event.
- Switch the per-event access-check error log to structured klog.ErrorS
  to comply with the project Go style guide.
- Strengthen TestGet_WithAccess to assert the concrete *TenantNamespace
  type plus Name, Kind, and APIVersion, so type or metadata regressions
  fail fast.

Assisted-By: Claude <[email protected]>
Signed-off-by: Myasnikov Daniil <[email protected]>
(cherry picked from commit 61ed7ad)