Part of #14809

Implement support for the JSON Schema numeric validation keywords in Go code generated by mdatagen.

Scope

• minimum: Value must be >= the specified minimum.
• maximum: Value must be <= the specified maximum.
• exclusiveMinimum: Value must be > the specified bound.
• exclusiveMaximum: Value must be < the specified bound.

Acceptance Criteria

• Generated Validate() methods enforce all four numeric constraints.
• Validation errors clearly indicate the violated bound and allowed range.
• Unit tests cover boundary values and invalid inputs for each keyword.