Update .autoManagedVhd marker file location to agent work folder (#5416)

* Update the location of .autoManagedVhd marker file

* Address feedback

* Use executionContext.Variables

---------

Co-authored-by: Semih Okur <seokur@microsoft.com>

Author: sanjuyadav24

src/Agent.Plugins/GitSourceProvider.cs

@@ -618,12 +618,16 @@ public async Task GetSourceAsync(
                     }
                 }
 
-                if (File.Exists(Path.Combine(targetPath, ".autoManagedVhd"))
+                string agentWorkFolder = executionContext.Variables.GetValueOrDefault("agent.workfolder")?.Value;
+                if (!string.IsNullOrEmpty(agentWorkFolder)
+                    && File.Exists(Path.Combine(agentWorkFolder, ".autoManagedVhd"))
                     && !AgentKnobs.DisableAutoManagedVhdShallowOverride.GetValue(executionContext).AsBoolean())
                 {
-                    // The existing working directory comes from an auto-managed VHD and is a full, 
-                    // non-shallow clone of the repository. Some pipelines enable shallow fetch, but 
-                    // Git cannot convert an existing full clone into a shallow one in-place. 
+                    // The existing working directory comes from an AutoManagedVHD (indicated by the
+                    // .autoManagedVhd marker file placed in the agent work folder).
+                    // An AutoManagedVHD always contains a full, non-shallow clone of the repository.
+                    // Some pipelines enable shallow fetch parameters (e.g., fetchDepth > 0). However,
+                    // Git cannot convert an existing full clone into a shallow one in-place.
                     //
                     // Technical reason:
                     //   A full clone already has complete commit history and object reachability.