feat: map validation constraints to OpenAPI schema properties

[soulcodingmatt]

Task

Propagate Equilibrium validation annotations to corresponding OpenAPI schema constraints.

Parent story: #12
Parent epic: #18

Details

Map validation annotations to OpenAPI properties:

• @NotNull → field in required array
• @NotBlank → minLength: 1
• @Size(min, max) → minLength/maxLength (strings) or minItems/maxItems (arrays)
• @Min(n) → minimum: n
• @Max(n) → maximum: n
• @Pattern(regexp) → pattern: regexp
• @Email → format: email
• @Positive → exclusiveMinimum: 0
• @Negative → exclusiveMaximum: 0

Acceptance Criteria

• Each supported validation annotation maps to the correct OpenAPI property
• Multiple constraints on the same field are combined correctly
• Unsupported annotations are silently ignored (no errors)
• Unit tests cover each constraint mapping