Opt pooled async methods out of runtime async

[jakobbotsch]

We do not have a replacement for this concept in runtime async (yet) and ASP.NET is seeing some regression from that. For now opt these methods out of runtime async.

cc @BrennanConroy

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[Copilot]

Pull request overview

This PR introduces an internal opt-out mechanism for “runtime async” generation and applies it to a set of pooled-async (PoolingAsyncValueTaskMethodBuilder*) methods across core libraries and key networking/IO components to avoid regressions when runtime async is enabled.

Changes:

• Add RuntimeAsyncMethodGenerationAttribute (internal, shared source) to control runtime-async method generation.
• Annotate selected pooled-async methods with [RuntimeAsyncMethodGeneration(false)] in core IO, pipelines, and System.Net implementations.
• Wire the shared attribute source into multiple library projects so the annotation is available at compile time.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/IO/Stream.cs	Opts ReadAtLeastAsyncCore (pooled async) out of runtime async generation.
src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs	Opts ReadAsyncSlowPath / WriteAsyncSlowPath pooled async paths out of runtime async generation.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Includes the new shared attribute source in CoreLib (link path needs adjustment).
src/libraries/System.Net.WebSockets/src/System/Net/WebSockets/ManagedWebSocket.cs	Opts key pooled async receive/buffering helpers out of runtime async generation.
src/libraries/System.Net.WebSockets/src/System.Net.WebSockets.csproj	Includes the shared attribute source file in the project.
src/libraries/System.Net.Security/src/System/Net/Security/SslStream.IO.cs	Opts pooled async TLS frame/read helpers out of runtime async generation.
src/libraries/System.Net.Security/src/System.Net.Security.csproj	Includes the shared attribute source file in the project.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/RawConnectionStream.cs	Opts pooled async ReadAsync override out of runtime async generation.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnection.cs	Opts pooled async ReadBufferedAsyncCore out of runtime async generation.
src/libraries/System.Net.Http/src/System.Net.Http.csproj	Includes the shared attribute source file in the project.
src/libraries/System.IO.Pipes/src/System.IO.Pipes.csproj	Includes the shared attribute source file in the project.
src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/StreamPipeWriter.cs	Opts pooled async flush core out of runtime async generation (NET-only section).
src/libraries/System.IO.Pipelines/src/System/IO/Pipelines/StreamPipeReader.cs	Opts pooled async local core reader loop out of runtime async generation (NET-only section).
src/libraries/System.IO.Pipelines/src/System.IO.Pipelines.csproj	Includes the shared attribute source file for .NETCoreApp builds.
src/libraries/Common/src/System/Threading/AsyncOverSyncWithIoCancellation.cs	Opts pooled async work-item wrappers out of runtime async generation.
src/libraries/Common/src/System/Runtime/CompilerServices/RuntimeAsyncMethodGenerationAttribute.cs	Adds the shared internal attribute definition (minor implementation/style issues noted).

[teo-tsirpanis]

I would expect that a custom async method builder would imply opting out of runtime async by itself.