chore(core): fix deprecation warnings

[hgw77]

Summary

This PR fixes all deprecation warnings in the test suite to ensure compatibility with Rails 7.1+ and prepare for the upcoming Rails 7.2 release. All 1452 tests continue to pass with zero failures.

Changes Made

• Migrated from deprecated secrets.yml to Rails 7.1+ configuration
  • Renamed config/secrets.yml to config/secrets.yml.deprecated
  • Created config/initializers/secret_key_base.rb to set secret_key_base via environment variables (Rails 7.1+ recommended approach)
  • Production continues to use MONSOON_RAILS_SECRET_TOKEN environment variable
  • Test and development environments use the same secure keys as before
• Updated fixture path configuration to use plural form
  • Changed config.fixture_path to config.fixture_paths (array) in spec/spec_helper.rb
  • Updated plugins/metrics/test/test_helper.rb to use fixture_paths instead of deprecated singular form
• Modernized RSpec syntax in authentication specs
  • Replaced deprecated .stub() syntax with modern allow().to receive() in plugins/monsoon-openstack-auth/spec/lib/monsoon_openstack_auth/authentication/auth_session_spec.rb
  • Updated 8 configuration stub blocks and 2 double object stubs
  • Replaced Rails.application.secrets.secret_key_base with Rails.application.secret_key_base in plugins/monsoon-openstack-auth/lib/monsoon_openstack_auth/authentication/auth_session.rb
• Updated web console configuration
  • Changed config.web_console.whitelisted_ips to config.web_console.allowed_ips in config/environments/development.rb

Checklist

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have made corresponding changes to the documentation (if applicable).
• My changes generate no new warnings or errors.