Allow object-aware external dereference component keys

[ayushshrivastv]

Closes #438

Summary

• add an object-aware ExternalLoader.componentKey overload in both OpenAPIKit variants
• keep the URL-only key path as the compatibility fallback so loaders can implement either form
• compute external reference component keys after loading so the decoded value can inform the key
• add regression tests covering the object-aware override path and the default compatibility path

Testing

• swift test --filter JSONReferenceTests
• swift test --filter ExternalDereferencingDocumentTests

[ayushshrivastv]

Follow-up on the CI failures for this branch.

What changed:

• Replaced the async let fan-out in both Path Item/DereferencedPathItem.swift implementations with sequential awaits.
• Replaced the task-group based concurrent traversal in the shared collection external-dereferencing helpers with sequential traversal:
  • Array+ExternallyDereferenceable.swift
  • Dictionary+ExternallyDereferenceable.swift
  • OrderedDictionary+ExternallyDereferenceable.swift
  • in both OpenAPIKit and OpenAPIKit30

Why:

• The failing jobs were not hitting the new object-aware ExternalLoader.componentKey behavior itself.
• CI was crashing in ExternalDereferencingDocumentTests.test_example on Linux/nightly paths while external references were being loaded concurrently.
• The stack traces consistently pointed into the external dereferencing pipeline during parallel loads, including PathItem dereferencing and the shared collection helpers that fan out work across arrays and dictionaries.
• Serializing those dereference/load paths avoids the runtime instability while preserving the behavior of the feature change in this PR.

What I verified locally:

• swift test --filter ExternalDereferencingDocumentTests -Xswiftc -strict-concurrency=complete
• swift test --filter JSONReferenceTests -Xswiftc -strict-concurrency=complete
• git diff --check