Fix: Replace Process.waitUntilExit() with DispatchGroup to prevent hangs

The waitUntilExit() call was causing hangs when invoked from Swift concurrency's
cooperative thread pool. This is because waitUntilExit() runs the current thread's
runloop in NSDefaultRunLoopMode while waiting, which can cause unexpected behavior
when called from arbitrary threads or thread pool contexts.

The issue occurs because:
1. waitUntilExit() runs the runloop in default mode, allowing timers and callbacks
   to fire unexpectedly (see Mike Ash's article on dangerous Cocoa calls)
2. Swift concurrency's cooperative thread pool doesn't guarantee which thread
   executes a given task, making runloop-dependent APIs unreliable
3. The runloop may not be properly configured on thread pool threads

The fix uses a DispatchGroup that is entered before starting the process and
left in the process's terminationHandler. The close() method then calls wait()
on the group, which is a simple blocking call without runloop involvement.

This approach is safe because:
- It doesn't depend on runloop infrastructure
- It works correctly regardless of which thread it's called from
- It provides clean synchronization with process termination

References:
- https://stackoverflow.com/questions/34996937/how-to-safely-use-nstask-waituntilexit-off-the-main-thread
- https://mikeash.com/pyblog/friday-qa-2009-11-13-dangerous-cocoa-calls.html

Author: johnliedtke

Sources/PklSwift/MessageTransport.swift

@@ -127,6 +127,8 @@ public class ServerMessageTransport: BaseMessageTransport, @unchecked Sendable {
     var process: Process?
     let pklCommand: [String]?
 
+    private let processTerminationGroup = DispatchGroup()
+
     override var running: Bool { self.process?.isRunning == true }
 
     override convenience init() {
@@ -153,6 +155,10 @@ public class ServerMessageTransport: BaseMessageTransport, @unchecked Sendable {
         self.process!.standardInput = self.writer
         let debugArguments = arguments
         debug("Spawning command \(pklCommand[0]) with arguments \(debugArguments)")
+        self.processTerminationGroup.enter()
+        self.process?.terminationHandler = { [processTerminationGroup] _ in
+            processTerminationGroup.leave()
+        }
         try self.process!.run()
     }
 
@@ -186,7 +192,7 @@ public class ServerMessageTransport: BaseMessageTransport, @unchecked Sendable {
         #else
         self.process?.terminate()
         #endif
-        self.process!.waitUntilExit()
+        self.processTerminationGroup.wait()
         self.process = nil
     }