Add path property to ValidationBuilder for path override

[dhoepelman]

Fixes #198

Summary

Adds support for overriding validation paths to suppress redundant path segments, particularly useful for inline/value classes serialized with kotlinx.serialization.

Implementation

• Added var path: ValidationPath? property to ValidationBuilder
• Modified validate(), ifPresent(), required(), and onEach*() methods to respect the path override
• When path is set (e.g., to ValidationPath.EMPTY), it replaces the automatic path segment from the property

Usage Example

@JvmInline
value class ValueClass(val integer: Int)

data class WrapperClass(val valueClass: ValueClass)

val validation = Validation<WrapperClass> {
  WrapperClass::valueClass {
    ValueClass::integer {
      path = ValidationPath.EMPTY  // Suppresses .integer from the path
      minimum(1)
    }
  }
}

// Error path will be: .valueClass (not .valueClass.integer)

This is particularly useful when value classes are transparently serialized by kotlinx.serialization, so the JSON path doesn't include the inner property.

Testing

• Added comprehensive test suite in PathOverrideTest.kt
• All existing tests pass
• API compatibility maintained (new optional property)

Notes

• Follows the same pattern as RequiredValidationBuilder.hint
• The path property is nullable to distinguish "not set" from "set to EMPTY"
• Works with nested validations and doesn't affect sibling validations