Add patternProperties support to JSON Schema objects

[ayushshrivastv]

Closes #391

Summary

• add patternProperties support to OpenAPI 3.1 object schemas in OpenAPIKit
• thread patternProperties through encoding/decoding, dereferencing, simplification, and schema combining
• add regression coverage for object-schema coding, dereferenced schemas, and schema fragment combining

Testing

• swift test --filter DereferencedSchemaObjectTests -Xswiftc -strict-concurrency=complete
• swift test --filter SchemaFragmentCombiningTests -Xswiftc -strict-concurrency=complete
• swift test --filter SchemaObjectTests -Xswiftc -strict-concurrency=complete

[ayushshrivastv]

Nightly and coverage failures here were coming from Linux compiler/resource pressure rather than the patternProperties change itself.

What was happening:

• swiftlang/swift:nightly-focal was failing in the build step with compile command failed due to signal 9.
• codecov was also hitting a Linux resource ceiling during the full coverage run.

What I changed in follow-ups:

• reduced the nightly job's compiler memory pressure in .github/workflows/tests.yml by using -j 1, -disable-batch-mode, and no debug info for swiftlang/swift:nightly-focal
• reduced coverage-job build pressure in .github/workflows/codecov.yml by forcing -j 1
• tightened AnyCodingKey in Sources/OpenAPIKitCore/OrderedDictionary/OrderedDictionary.swift and marked it @unchecked Sendable to eliminate the repeated nightly Sendable warning coming from that helper

I also reproduced the nightly failure locally in swiftlang/swift:nightly-focal before pushing the mitigation.