jextract: deduplicate protocol default JNI thunks

[Hokila]

Summary

• Deduplicate existential-box JNI dispatch thunks by their complete JNI C symbol name.
• Keep the first thunk and preserve the existing Java API and dispatch path.
• Share JNI parameter/signature construction between deduplication and emission.
• Add a regression test for a protocol requirement with a default implementation.

Fixes #891

Testing

• swift test --filter JNIProtocolTests (18 tests)
• swift test --filter JExtractSwiftTests (578 tests, 73 suites)
• xcrun swift-format lint --configuration .swift-format on changed Swift files
• Generated fixture Swift passed swiftc -typecheck

The full test suite remains environment-blocked for Java/Gradle tests because no Java Runtime is installed.