feat: support OpenAPI 3.2 additional operations

[cmmoran]

What/Why/How?

Add support for OpenAPI 3.2 Path Item additionalOperations, including custom HTTP methods and webhook/callback modeling needed to expose those operations in Redoc. Standard OpenAPI operations retain their existing behavior.

The implementation keeps method names case-sensitive for custom operations, updates operation discovery and menu construction, and adds regression coverage for parsing and rendering behavior.

Reference

• OpenAPI 3.2 Path Item Object

Tests

• npm test
• npm run lint (0 errors; one existing warning in RedocStandalone.tsx)
• Prettier check on changed files

Screenshots (optional)

Not applicable; this draft focuses on operation discovery and rendering behavior.

Check yourself

• Code is linted
• Tested
• All new/updated code is covered with tests