fix: resolve Elixir 1.20 compilation warnings

[gilbertwong96]

Resolve all compilation warnings in Elixir 1.20.

• Remove redundant resolve_schema_modules_from_media_type/2 clause
• Fix description || "" fallback in controller.ex
• Fix always-nil yaml_encoder() check
• Remove unreachable allOf: [] and oneOf: [] cast clauses in deprecated_cast.ex
• Remove unreachable term_type/1 nil guard and validate_required_properties nil guard clauses