feat: generate OpenAPI YAML schema files from DTO metadata

[soulcodingmatt]

Task

Implement the YAML schema file generation that produces valid OpenAPI component schemas from processed DTO metadata.

Parent story: #12
Parent epic: #18

Details

• Use the type mapping to produce a complete OpenAPI schema component
• Generate a .yaml file per DTO in the configured output directory
• Include required array for non-nullable fields
• Include description from Javadoc if available
• Validate generated YAML is syntactically correct

Acceptance Criteria

• Given a simple DTO with String and int fields, a valid .yaml schema file is generated
• Given a DTO with @NotNull fields, the required array is populated correctly
• Given multiple DTOs, separate schema files are generated for each
• Generated schemas pass OpenAPI schema validation
• Compile-testing harness test verifies end-to-end generation