Support configurable Windows Job Object assignment

On Windows, processes are currently assigned to Job Objects
unconditionally. The drawback to this is that assignment can fail when
the parent is already in a Job Object that doesn't allow nesting, or
when the system otherwise cannot form a valid hierarchy.

Add `PlatformOptions.JobObjectAssignment`, describing how Job Object
assignment is handled. The default behavior of `.always` matches the
current behavior.

Author: broken-circle

Sources/Subprocess/Error.swift

@@ -112,7 +112,7 @@ extension SubprocessError {
     private enum Context: Sendable, Hashable {
         case string(String)
         case int(Int)
-        case processControlOperation(ProcessControlOperation)
+        case processControlOperation(ProcessControlOperation, reason: String?)
     }
 
     internal enum ProcessControlOperation: Sendable, Hashable {
@@ -199,22 +199,26 @@ extension SubprocessError: CustomStringConvertible, CustomDebugStringConvertible
             }
 
         case .processControlFailed:
-            if let context = self.context[self.code],
-                case .processControlOperation(let operation) = context
-            {
-                switch operation {
-                case .sendSignal(let signal):
-                    return "Failed to send signal \(signal) to child process"
-                case .terminate:
-                    return "Failed to terminate child process."
-                case .suspend:
-                    return "Failed to suspend child process."
-                case .resume:
-                    return "Failed to resume child process."
-                }
-            } else {
+            guard let context = self.context[self.code],
+                case .processControlOperation(let operation, let reason) = context
+            else {
                 return "Failed to control child process state"
             }
+            var message: [String]
+            switch operation {
+            case .sendSignal(let signal):
+                message = ["Failed to send signal \(signal) to child process."]
+            case .terminate:
+                message = ["Failed to terminate child process."]
+            case .suspend:
+                message = ["Failed to suspend child process."]
+            case .resume:
+                message = ["Failed to resume child process."]
+            }
+            if let reason {
+                message.append("Reason: \(reason)")
+            }
+            return message.joined(separator: " ")
         }
     }
 
@@ -290,11 +294,15 @@ extension SubprocessError {
         )
     }
 
-    internal static func processControlFailed(_ operation: ProcessControlOperation, underlyingError: UnderlyingError?) -> Self {
+    internal static func processControlFailed(
+        _ operation: ProcessControlOperation,
+        reason: String? = nil,
+        underlyingError: UnderlyingError?
+    ) -> Self {
         return SubprocessError(
             code: .processControlFailed,
             underlyingError: underlyingError,
-            context: [.processControlFailed: .processControlOperation(operation)]
+            context: [.processControlFailed: .processControlOperation(operation, reason: reason)]
         )
     }