jextract/jni: Actor properties with synchronous access are imported as async #942

Problem

JNI extraction currently treats actor property accessors as implicitly async even when the property is explicitly nonisolated. It also misses the synchronous access path provided by an immutable actor property that witnesses a nonisolated protocol getter.

Example

public protocol Identified {
  var identifier: Int64 { get }
}

public actor Record: Identified {
  public let identifier: Int64
  public nonisolated var label: String { "record" }

  public init(identifier: Int64) {
    self.identifier = identifier
  }
}

Both getters can be exposed synchronously, so Java callers should be able to write:

long identifier = record.getIdentifier();
String label = record.getLabel();

Currently, both getters return CompletableFuture values.

For identifier, removing await from the generated thunk is not sufficient: outside the actor's defining module, direct access to the stored property remains isolated. Synchronous access is available through the protocol requirement, for example:

(record as any Identified).identifier

Reference workaround implementation: qoncept#4