Implement core validation logic with type coercion

[matapatos]

Implement core validation logic with type coercion

Changes

• Add validate_type() function for type validation and coercion
• Add validate_field_value() implementation with nullable type support
• Add add_field_errorf() for formatted error messages with type information
• Support coercion from string to int/long
• Support coercion from string/long to double
• Support coercion from int/float to string
• Support coercion from string/int/float to bool
• Support coercion from object to array
• Handle nullable types (e.g., ?string)
• Respect strict mode from ModelConfigs
• Update function signatures to pass properties parameter

Impact

• Core validation logic is now functional
• Type coercion works for primitive types
• Better error messages with expected vs actual types
• Nullable types are properly handled

Closes #2