Fix compilation errors: move BackgroundThreadJob to ChainableAst and use compound assignment

- Moved BackgroundThreadJob property from PipelineAst to ChainableAst base class
  to make it accessible to both PipelineAst and PipelineChainAst
- Updated PipelineChainAst.Copy() to preserve BackgroundThreadJob property
- Fixed IDE0074 warning by using ??= compound assignment operator
- Fixes CS1061 error where PipelineBaseAst didn't have BackgroundThreadJob property

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

Author: Copilot

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

@@ -5540,6 +5540,11 @@ 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>
@@ -5610,7 +5615,9 @@ public PipelineChainAst(
         /// </returns>
         public override Ast Copy()
         {
-            return new PipelineChainAst(Extent, CopyElement(LhsPipelineChain), CopyElement(RhsPipeline), Operator, Background);
+            var copy = new PipelineChainAst(Extent, CopyElement(LhsPipelineChain), CopyElement(RhsPipeline), Operator, Background);
+            copy.BackgroundThreadJob = this.BackgroundThreadJob;
+            return copy;
         }
 
         internal override object Accept(ICustomAstVisitor visitor)
@@ -5767,11 +5774,6 @@ public PipelineAst(IScriptExtent extent, CommandBaseAst commandAst) : this(exten
         /// </summary>
         public bool Background { get; internal set; }
 
-        /// <summary>
-        /// Indicates that this pipeline should be run in the background as a ThreadJob.
-        /// </summary>
-        public bool BackgroundThreadJob { get; internal set; }
-
         /// <summary>
         /// If the pipeline represents a pure expression, the expression is returned, otherwise null is returned.
         /// </summary>

src/System.Management.Automation/engine/runtime/Operations/MiscOps.cs

@@ -593,11 +593,8 @@ internal static void InvokePipelineInBackground(
                     try
                     {
                         commandInfo = context.SessionState.InvokeCommand.GetCmdlet("Start-ThreadJob");
-                        if (commandInfo == null)
-                        {
-                            // Fall back to Start-Job if Start-ThreadJob is not available
-                            commandInfo = new CmdletInfo("Start-Job", typeof(StartJobCommand));
-                        }
+                        // Fall back to Start-Job if Start-ThreadJob is not available
+                        commandInfo ??= new CmdletInfo("Start-Job", typeof(StartJobCommand));
                     }
                     catch (CommandNotFoundException)
                     {