[release/8.0-preview4] [wasm] GenerateWasmBootJson - close file handles

[github-actions]

Backport of #85480 to release/8.0-preview4

/cc @lewing @radical

Customer Impact

Blazor hosted templates will fail to publish during the nested build process.

Expected to fix the failures found in dotnet/aspnetcore#47540 (comment) .

Background

• this was found when looking at the test failures mentioned in Update SDK aspnetcore#47540 (comment) .

The hypothesis is that the following operations happen in order:

1. GenerateBlazorBootExtensionJson calls File.Create to create obj/Release/net8.0/blazor.publish.boot-extension.json, and the file handle is closed
2. GenerateWasmBootJson opens the same file with File.OpenRead but does not close it
3. GenerateBlazorBootExtensionJson runs again as part of the nested build, but fails because (2) left the file open

The "GenerateBlazorBootExtensionJson" task failed unexpectedly. [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
System.IO.IOException: The process cannot access the file '/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/obj/Release/net8.0/blazor.publish.boot-extension.json' because it is being used by another process. [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Init(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Int64& fileLength, UnixFileMode& filePermissions) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Func`4 createOpenException) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at System.IO.File.Create(String path) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.NET.Sdk.BlazorWebAssembly.GenerateBlazorBootExtensionJson.Execute() [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]

Testing

Automated and manual verification that the change is correct and doesn't regress working scenarios.

Risk

Very low, the change is very small and tested

[lewing]

@carlossanlop the failures look fine and this is wasm only, please merge asap