Add URI resolution helpers for base URI resolution

[ayushshrivastv]

Summary

• add reusable URI resolution helpers for raw URLs, JSON references, and documents in both OpenAPIKit and OpenAPIKit30
• add schema base URI helpers and identifier support on OpenAPIKit schema contexts so callers can compose parent base URIs with schema identifiers
• cover the new helpers with focused document, reference, and schema URI resolution tests

Closes #439.

Testing

• swift test --filter JSONReferenceURIResolutionTests
• swift test --filter DocumentURIResolutionTests
• swift test --filter JSONSchemaURIResolutionTests
• git diff --check

[ayushshrivastv]

Implementation notes for reviewers:

• Added shared URI-resolution helpers in both modules so callers can resolve URI references without reimplementing base-URI logic. JSONReference now exposes uriReference and resolvedURI(relativeTo:), and OpenAPI.Reference in OpenAPIKit forwards to the same logic.
• Added document-level convenience helpers. In OpenAPIKit, OpenAPI.Document.baseURI(relativeTo:) establishes the document base URI by combining the retrieval URI with $self when present. In OpenAPIKit30, the retrieval URI is used directly since 3.0 documents do not support $self.
• Added a small shared URL helper in OpenAPIKitCore so relative and fragment-only URI references are resolved consistently from one place.
• Added schema base-URI support through JSONSchemaContext.baseURI(relativeTo:), and added id support on the OpenAPIKit schema context so schema identifiers can participate in base-URI composition through the public API.
• Added focused regression coverage for:
  • JSONReferenceURIResolutionTests
  • DocumentURIResolutionTests
  • JSONSchemaURIResolutionTests

Files with the main implementation:

• Sources/OpenAPIKitCore/Utility/URL+Resolution.swift
• Sources/OpenAPIKit/JSONReference+URIResolution.swift
• Sources/OpenAPIKit30/JSONReference+URIResolution.swift
• Sources/OpenAPIKit/Document/Document+URIResolution.swift
• Sources/OpenAPIKit30/Document/Document+URIResolution.swift
• Sources/OpenAPIKit/Schema Object/JSONSchemaContext.swift
• Sources/OpenAPIKit/Schema Object/JSONSchema.swift
• Sources/OpenAPIKit30/Schema Object/JSONSchemaContext.swift

Local verification run on this branch:

• swift test --filter JSONReferenceURIResolutionTests
• swift test --filter DocumentURIResolutionTests
• swift test --filter JSONSchemaURIResolutionTests
• git diff --check

[ayushshrivastv]

Follow-up for the failing linux (swift:6.1-jammy) job: the failure was not in the new URI-resolution code. It was the same older Swift async runtime crash in OpenAPI.PathItem.externallyDereferenced / OpenAPIKit30/Path Item/DereferencedPathItem.swift during ExternalDereferencingDocumentTests.test_example. I cherry-picked the proven compatibility fix (bd82590) onto this branch, pushed it, and re-ran the relevant coverage locally.

Additional local verification after the follow-up commit:

• swift test --filter ExternalDereferencingDocumentTests/test_example -Xswiftc -strict-concurrency=complete
• swift test --filter JSONReferenceURIResolutionTests
• swift test --filter DocumentURIResolutionTests
• swift test --filter JSONSchemaURIResolutionTests
• git diff --check