feat: Add test endpoints for Container Virtual Registry upstreams

Relates to issue #578679

Changes

This MR implements two test endpoints for Container Virtual Registry upstreams to achieve parity with Maven Virtual Registry API endpoints.

API Endpoints Added

1. POST /groups/:id/-/virtual_registries/container/upstreams/test

   • Tests connection before creating an upstream
   • Validates URL and credentials (username/password with all_or_none_of constraint)
   • Creates temporary upstream instance without persisting
   • Returns test results immediately

2. GET /virtual_registries/container/upstreams/:id/test

   • Tests connection to an existing upstream
   • Requires read_virtual_registry authorization
   • Calls the upstream's test method and returns results

Model Enhancement

• Added test method to VirtualRegistries::Container::Upstream model
• Implements HEAD request validation using bearer token authentication
• Returns { success: true } for 2XX/404 responses
• Returns { success: false, result: "Error: ..." } for other status codes or HTTP errors
• Uses existing cache entries' relative paths for testing, or empty path for new records

Test Coverage

• Created comprehensive RSpec tests for both endpoints (ee/spec/requests/api/virtual_registries/container/upstreams_test_spec.rb)
• Tests valid/invalid parameters, authentication, authorization, feature flag behavior, and different user roles

Documentation

• Added "Test connection before creating an upstream registry" section
• Added "Test connection to an upstream registry" section
• Includes example requests/responses and notes about 2XX/404 success interpretation
• Documented as GitLab 18.7

Key Implementation Details

• Pattern Consistency: Follows the exact pattern from Maven Virtual Registry implementation
• Authentication: Uses bearer token authentication via the existing headers method
• Authorization: Both endpoints require read_virtual_registry permission
• Feature Flag: Uses existing container_virtual_registries feature flag
• Error Handling: Properly handles HTTP errors and returns structured error messages