feat: implement Java-to-OpenAPI type mapping

[soulcodingmatt]

Task

Implement the mapping from Java field types to OpenAPI schema types.

Parent story: #12
Parent epic: #18

Details

Map these Java types to OpenAPI types:

• String → type: string
• int/Integer, long/Long → type: integer
• double/Double, float/Float, BigDecimal → type: number
• boolean/Boolean → type: boolean
• LocalDate → type: string, format: date
• LocalDateTime/Instant → type: string, format: date-time
• List<T> → type: array, items: <T>
• Set<T> → type: array, items: <T>, uniqueItems: true
• Map<K,V> → type: object, additionalProperties: <V>
• Nested DTOs → $ref: '#/components/schemas/NestedDtoName'

Acceptance Criteria

• All primitive and wrapper types are correctly mapped
• Collection types (List, Set, Map) are correctly mapped
• Date/time types use correct OpenAPI format strings
• Nested DTO references use $ref
• Unit tests cover all type mappings