Move BackgroundThreadJob property to PipelineBaseAst base class

- Moved BackgroundThreadJob from ChainableAst to PipelineBaseAst
- This ensures the property is accessible when the parameter type is PipelineBaseAst
- Fixes CS1061 compilation error where PipelineBaseAst didn't have BackgroundThreadJob

Co-authored-by: kilasuit <6355225+kilasuit@users.noreply.github.com>

Author: Copilot

src/System.Management.Automation/engine/parser/ast.cs

@@ -5540,11 +5540,6 @@ public abstract class ChainableAst : PipelineBaseAst
         protected ChainableAst(IScriptExtent extent) : base(extent)
         {
         }
-
-        /// <summary>
-        /// Indicates that this pipeline should be run in the background as a ThreadJob.
-        /// </summary>
-        public bool BackgroundThreadJob { get; internal set; }
     }
 
     /// <summary>
@@ -5682,6 +5677,11 @@ public virtual ExpressionAst GetPureExpression()
         {
             return null;
         }
+
+        /// <summary>
+        /// Indicates that this pipeline should be run in the background as a ThreadJob.
+        /// </summary>
+        public bool BackgroundThreadJob { get; internal set; }
     }
 
     /// <summary>