Preserve security definitions in bundled OpenAPI output

[Vibaswan]

Summary

• The bundler was silently dropping the root-level security field and components.securitySchemes when bundling multi-file OpenAPI specs. This PR fixes that.
• Added "security" to the list of top-level keys copied from source YAML files in _process_yaml_object.
• Initialized securitySchemes: {} in the default components structure and added name validation for security schemes, consistent with how schemas and responses are handled.
• Fixed a crash in _check_nested_components when a components block contains only securitySchemes and no schemas key.

Test plan

• Added openapiart/tests/security/security.yaml — a fixture covering bearer, API key, OAuth2, and OpenID Connect schemes, plus root-level and operation-level security overrides.
• Added openapiart/tests/test_security_definitions.py — verifies that after bundling, the output YAML contains the correct root security list, all four securitySchemes entries with their full structure, and per-operation security overrides (/status with OAuth2 scope, /public with empty list).

Note: no validation for the security schemas has been added

[lthompson-keysight]

LGTM

[lthompson-keysight]

LGTM