JNI: support Foundation values in protocol callbacks - #5

Problem

JNI protocol callbacks reject Foundation value types even though the Java-facing method signatures already support them. As a result, a Swift protocol accepting or returning Data, Date, URL, or UUID cannot be implemented as an ordinary Java callback.

For example:

import Foundation

public protocol IdentityCallback {
  func transform(_ value: UUID) -> UUID
}

public func apply(_ callback: some IdentityCallback, _ value: UUID) -> UUID {
  callback.transform(value)
}

With this change, Java can implement that protocol using the existing java.util.UUID representation:

IdentityCallback callback = value ->
    java.util.UUID.fromString("87654321-4321-8765-cba9-876543210fed");
java.util.UUID result = MySwiftLibrary.apply(callback, input);

The generated callback bridge converts the Swift argument into a Java UUID and converts the Java result back into Swift. Data, Date, and URL callbacks instead use their existing generated wrapper classes and native value storage.

Changes

• Allow Data, Date, and URL in callback parameters and results using the existing wrapper conversions.
• Convert UUID through java.util.UUID.fromString and toString, consistently with the existing JNI-facing UUID API. UUIDs must not be treated as opaque Swift pointer wrappers.
• Add the java.util.UUID projection and dependency mapping to runtime support so wrap-java can generate callback methods with UUID signatures.
• Cover both Foundation and FoundationEssentials, plus Optional Data and Date, in generator regressions.
• Add Java runtime tests for all four value types, a Java callback returning a different UUID, and present/empty Optional values. Document the supported representations and current limits.

Scope

This change enables Foundation values within the types already handled by JNI method extraction. Arrays of Foundation values and Optional URL/UUID parameters need separate extraction support and are outside this PR.

Validation

• Confirmed the scalar callback generator regressions fail before the fix.
• swift test --disable-experimental-prebuilts --disable-sandbox: passed.
• ./gradlew :Samples:SwiftJavaExtractJNISampleApp:test --no-daemon: passed (278 Java tests, no failures or skips), including the five new runtime tests.
• swift-format lint --strict on changed Swift files and git diff --check: passed.

Tested with Apple Swift 6.4 and JetBrains Runtime 25.0.2 on macOS.