[CmdPal] Fix apps using incorrect AsSpan usage after CsWin32 upgrade (#40156)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
Root Cause:
After the CsWin32 upgrade, the code switched from using manually
allocated unmanaged buffers (Marshal.AllocHGlobal) to using Span<char>
via outBuffer.AsSpan(). However, the Span length passed to
SHLoadIndirectString was not correctly calculated.

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## PR Checklist

- [ ] **Closes:** #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Apps/Programs/UWPApplication.cs

@@ -218,13 +218,12 @@ public class UWPApplication : IProgram
                 return string.Empty;
             }
 
-            var capacity = 1024U;
-            PWSTR outBuffer = new PWSTR((char*)(void*)Marshal.AllocHGlobal((int)capacity * sizeof(char)));
+            Span<char> outBuffer = stackalloc char[1024];
             var source = $"@{{{packageFullName}? {parsed}}}";
 
             try
             {
-                PInvoke.SHLoadIndirectString(source, outBuffer.AsSpan()).ThrowOnFailure();
+                PInvoke.SHLoadIndirectString(source, outBuffer).ThrowOnFailure();
 
                 var loaded = outBuffer.ToString();
                 return string.IsNullOrEmpty(loaded) ? string.Empty : loaded;
@@ -234,7 +233,7 @@ public class UWPApplication : IProgram
                 try
                 {
                     var sourceFallback = $"@{{{packageFullName}?{parsedFallback}}}";
-                    PInvoke.SHLoadIndirectString(sourceFallback, outBuffer.AsSpan()).ThrowOnFailure();
+                    PInvoke.SHLoadIndirectString(sourceFallback, outBuffer).ThrowOnFailure();
                     var loaded = outBuffer.ToString();
                     return string.IsNullOrEmpty(loaded) ? string.Empty : loaded;
                 }
@@ -243,13 +242,6 @@ public class UWPApplication : IProgram
                     // ProgramLogger.Exception($"Unable to load resource {resourceReference} from {packageFullName}", new InvalidOperationException(), GetType(), packageFullName);
                     return string.Empty;
                 }
-                finally
-                {
-                }
-            }
-            finally
-            {
-                Marshal.FreeHGlobal((IntPtr)outBuffer.Value);
             }
         }
         else