Fix disk space issue in .NET PR creation by implementing sparse checkout (#9082)

The `Submit-AzureSdkForNetPr.ps1` script was performing a full clone of
`azure-sdk-for-net` (several GB), causing the pipeline to run out of
disk space and timeout.

## Changes

- Replace full `git clone` with sparse checkout using cone mode
- Fetch only the `eng/` directory (contains `Packages.Data.props` and
`packages/http-client-csharp/`)
- Use shallow fetch (`--depth 1`) to minimize history

```powershell
# Before: Full clone (~several GB)
git clone "https://github.com/$RepoOwner/$RepoName.git" $tempDir

# After: Sparse checkout (~185MB)
git init $tempDir
git remote add origin "https://github.com/$RepoOwner/$RepoName.git"
git sparse-checkout init --cone
git sparse-checkout set eng/packages/http-client-csharp eng
git fetch --depth 1 origin $BaseBranch
git checkout $BaseBranch
```

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Update .NET PR failing</issue_title>
> <issue_description>For the last several runs, the update .NET PR
script has timed out:
> ##[error]System.IO.IOException: No space left on device :
'/usr/local/vss-agent/4.264.2/_diag/Worker_20251123-215819-utc.log'
> at System.IO.RandomAccess.WriteAtOffset(SafeFileHandle handle,
ReadOnlySpan`1 buffer, Int64 fileOffset)
> at System.IO.StreamWriter.Flush(Boolean flushStream, Boolean
flushEncoder)
>    at System.Diagnostics.TextWriterTraceListener.Flush()
> at
Microsoft.VisualStudio.Services.Agent.HostTraceListener.WriteHeader(String
source, TraceEventType eventType, Int32 id) in
/mnt/vss/_work/1/s/src/Microsoft.VisualStudio.Services.Agent/HostTraceListener.cs:line
131
> at System.Diagnostics.TraceSource.TraceEvent(TraceEventType eventType,
Int32 id, String message)
> at Microsoft.VisualStudio.Services.Agent.Worker.Worker.RunAsync(String
pipeIn, String pipeOut) in
/mnt/vss/_work/1/s/src/Agent.Worker/Worker.cs:line 102
> at
Microsoft.VisualStudio.Services.Agent.Worker.Program.MainAsync(IHostContext
context, String[] args) in
/mnt/vss/_work/1/s/src/Agent.Worker/Program.cs:line 54
> System.IO.IOException: No 
> ,##[error]The job running on agent azsdk-pool 1 ran longer than the
maximum time of 60 minutes. For more information, see
https://go.microsoft.com/fwlink/?linkid=2077134
> ,##[warning]Agent azsdk-pool 1 did not respond to a cancelation
request with 00:05:00.
> Pool:
[azsdk-pool](https://dev.azure.com/azure-sdk/590cfd2a-581c-4dcb-a12e-6568ce786175/_settings/agentqueues?poolId=&queueId=532)
> Agent: azsdk-pool 1
> Started: Yesterday at 1:58 PM
> Duration: 1h 4m 59s
> 
> Job preparation parameters
> Parent pipeline used these runtime parameters
> 
>
https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/eng/pipeline/publish.yml#L126
> 
> When seeing the initial run it said the device was low on disk space.
We may need to do a sparse checkout instead.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #9081

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: JoshLove-msft <[email protected]>

Author: Copilot

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

@@ -70,14 +70,48 @@ New-Item -ItemType Directory -Path $tempDir -Force | Out-Null
 Write-Host "Created temp directory: $tempDir"
 
 try {
-    # Clone the repository
-    Write-Host "Cloning azure-sdk-for-net repository..."
-    git clone "https://github.com/$RepoOwner/$RepoName.git" $tempDir
+    # Use sparse checkout to clone only the necessary files
+    # This significantly reduces disk space usage as azure-sdk-for-net is a very large repository
+    Write-Host "Setting up sparse checkout for azure-sdk-for-net repository..."
+    
+    # Initialize empty git repository
+    git init $tempDir
     if ($LASTEXITCODE -ne 0) {
-        throw "Failed to clone repository"
+        throw "Failed to initialize repository"
     }
-
+    
     Push-Location $tempDir
+    
+    # Add the remote
+    git remote add origin "https://github.com/$RepoOwner/$RepoName.git"
+    if ($LASTEXITCODE -ne 0) {
+        throw "Failed to add remote"
+    }
+    
+    # Enable sparse checkout with cone mode for better performance
+    git sparse-checkout init --cone
+    if ($LASTEXITCODE -ne 0) {
+        throw "Failed to initialize sparse checkout"
+    }
+    
+    # Set the sparse checkout patterns - only the directories we need
+    git sparse-checkout set eng/packages/http-client-csharp eng
+    if ($LASTEXITCODE -ne 0) {
+        throw "Failed to set sparse checkout patterns"
+    }
+    
+    # Fetch only the main branch with depth 1
+    Write-Host "Fetching $BaseBranch branch with sparse checkout..."
+    git fetch --depth 1 origin $BaseBranch
+    if ($LASTEXITCODE -ne 0) {
+        throw "Failed to fetch repository"
+    }
+    
+    # Checkout the fetched branch
+    git checkout $BaseBranch
+    if ($LASTEXITCODE -ne 0) {
+        throw "Failed to checkout $BaseBranch"
+    }
 
     # Create a new branch
     Write-Host "Creating branch $PRBranch..."