Fix sparse checkout to include Azure.Core shared source files (#9092)

PR #9082 switched to sparse checkout but omitted
`sdk/core/Azure.Core/src/Shared`, causing build failures when Azure test
projects reference shared source files like `TrimmingAttribute.cs`,
`RawRequestUriBuilder.cs`, etc.

### Changes
- Added `sdk/core/Azure.Core/src/Shared` to sparse checkout pattern in
`Submit-AzureSdkForNetPr.ps1`

```powershell
git sparse-checkout set eng/packages/http-client-csharp eng sdk/core/Azure.Core/src/Shared
```

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Build is now failing in the Update PR</issue_title>
> <issue_description>#9082
updated to use sparse checkout but we are missing the Azure.Core shared
source files:
> 
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/TrimmingAttribute.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/example/basic/src/_Specs_.Azure.Example.Basic.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/RawRequestUriBuilder.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/client-generator-core/api-version/header/src/_Specs_.Azure.ClientGenerator.Core.ApiVersion.Header.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/TypeFormatters.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/client-generator-core/api-version/header/src/_Specs_.Azure.ClientGenerator.Core.ApiVersion.Header.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/RequestHeaderExtensions.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/client-generator-core/api-version/header/src/_Specs_.Azure.ClientGenerator.Core.ApiVersion.Header.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/AppContextSwitchHelper.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/client-generator-core/api-version/header/src/_Specs_.Azure.ClientGenerator.Core.ApiVersion.Header.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/ClientDiagnostics.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/client-generator-core/api-version/header/src/_Specs_.Azure.ClientGenerator.Core.ApiVersion.Header.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/DiagnosticScopeFactory.cs'
could not be found.
[/tmp/azure-sdk-for-net-20251125-171710/eng/packages/http-client-csharp/generator/TestProjects/Spector/http/azure/client-generator-core/api-version/header/src/_Specs_.Azure.ClientGenerator.Core.ApiVersion.Header.csproj::TargetFramework=net8.0]
> CSC : error CS2001: Source file
'/tmp/azure-sdk-for-net-20251125-171710/eng//../sdk/core/Azure.Core/src/Shared/DiagnosticScope.cs'
could not be found. [/tmp/azure-</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #9091

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Co-authored-by: JoshLove-msft <[email protected]>

Author: Copilot

packages/http-client-csharp/eng/scripts/Submit-AzureSdkForNetPr.ps1

@@ -95,7 +95,7 @@ try {
     }
 
     # Set the sparse checkout patterns - only the directories we need
-    git sparse-checkout set eng/packages/http-client-csharp eng
+    git sparse-checkout set eng/packages/http-client-csharp eng sdk/core/Azure.Core/src/Shared
     if ($LASTEXITCODE -ne 0) {
         throw "Failed to set sparse checkout patterns"
     }