Fix NuGet package missing Microsoft.Z3.dll due to inverted replace() logic (#8029)

* Initial plan

* Fix NuGet packaging and add GitHub Actions workflow

- Fixed critical bug in mk_nuget_task.py replace() function
- Created comprehensive GitHub Actions workflow for building NuGet packages

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

* Add documentation and improve code comments

- Added comprehensive README for NuGet build workflow
- Added detailed comments explaining the replace() function fix
- Verified all Python syntax is correct

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

scripts/mk_nuget_task.py

@@ -44,10 +44,20 @@ def classify_package(f, arch):
     return None
 
 def replace(src, dst):
+    """
+    Replace destination file with source file.
+    
+    Removes the destination file if it exists, then moves the source file to the destination.
+    This ensures that the file is always moved, whether or not the destination exists.
+    
+    Previous buggy implementation only moved when removal failed, causing files to be 
+    deleted but not replaced when the destination already existed.
+    """
     try:
         os.remove(dst)
     except:
-        shutil.move(src, dst)
+        pass
+    shutil.move(src, dst)
     
 def unpack(packages, symbols, arch):
     # unzip files in packages