Update OpenAPIKit dependency to 4.x

[cnaples79]

Summary

• Updates OpenAPIKit dependency from 3.3.0 to 4.0.0
• Updates Yams minimum version to 5.1.0 as required by OpenAPIKit v4
• Existing codebase is already compatible with OpenAPIKit v4 breaking changes

Changes Made

• Package.swift: Updated OpenAPIKit dependency from from: "3.3.0" to from: "4.0.0"
• Package.swift: Updated Yams dependency from "4.0.0"..<"7.0.0" to from: "5.1.0"

Compatibility Analysis

The codebase analysis shows existing code is already compatible with OpenAPIKit v4:

✅ Error Types: Already using GenericError (not the renamed InconsistencyError) ✅ OpenAPI Versions: Already supports v3.0.4 and v3.1.1 version cases ✅ Content Types: Already using contentTypes (plural) property where applicable
✅ AnyCodable: Usage patterns are compatible with v4 requirements ✅ Swift Version: Meets Swift 5.8+ requirement

Test Plan

• [ ] CI pipeline should validate compilation with new dependencies
• [ ] Existing tests should continue passing without modification
• [ ] Package resolution should succeed with OpenAPIKit 4.x and Yams 5.1+

This addresses the dependency update requested in issue #821, following the migration guide provided by the OpenAPIKit maintainer.

Fixes #821

[czechboy0]

Thank you for the PR, @cnaples79 - it might take me a few days to review and do some extra testing, just FYI.