Implement async handle support on Unix for FileStream

[Copilot]

Description

FileStream previously rejected non-blocking (O_NONBLOCK) handles on Unix. This PR lifts that restriction by adding proper poll-then-read/write support for non-blocking file descriptors, enabling FileStream to wrap async pipe/socket handles on Unix.

Native layer (pal_io.c, pal_io.h, entrypoints.c)

• Added SystemNative_ReadFromNonblocking and SystemNative_WriteToNonblocking
• Each attempts a read/write; on EAGAIN/EWOULDBLOCK, polls (indefinitely, on a thread pool thread) via Common_Poll until data/space is available or the pipe is closed, then retries the I/O
• EOF on read returns 0; closed pipe on write returns -1 with EPIPE

Interop (Interop.Read.cs, Interop.Write.cs)

• Added ReadFromNonblocking and WriteToNonblocking P/Invoke declarations

RandomAccess.Unix.cs

• ReadAtOffset and WriteAtOffset now check handle.IsAsync first and dispatch to the new non-blocking variants; the existing SupportsRandomAccess/pread/pwrite path is unchanged for synchronous handles

if (handle.IsAsync)
{
    result = Interop.Sys.ReadFromNonblocking(handle, bufPtr, buffer.Length);
}
else if (handle.SupportsRandomAccess)
{
    result = Interop.Sys.PRead(handle, bufPtr, buffer.Length, fileOffset);
    // ... ESPIPE fallback unchanged
}
else
{
    result = Interop.Sys.Read(handle, bufPtr, buffer.Length);
}

FileStream / tests

• Removed the ValidateHandle guard that threw ArgumentException for async handles on Unix
• Replaced the AsyncHandleOnUnix_FileStream_ctor_Throws test with SafeFileHandle_CreateAnonymousPipe_FileStream_SetsIsAsyncAndTransfersData, which verifies all four asyncRead/asyncWrite combinations work end-to-end using FileStream directly (no AnonymousPipeClientStream shim needed)

Changes

• src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs
• src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Unix.cs
• src/libraries/Common/src/Interop/Unix/System.Native/Interop.Read.cs
• src/libraries/Common/src/Interop/Unix/System.Native/Interop.Write.cs
• src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/File/OpenHandle.cs
• src/native/libs/System.Native/pal_io.c
• src/native/libs/System.Native/pal_io.h
• src/native/libs/System.Native/entrypoints.c

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[adamsitnik]

@stephentoub regarding #125220 (comment) this is what it would take to get FileStream support O_NONBLOCK on Unix.

[stephentoub]

If we move all of the async infrastructure from System.Net.Sockets into ThreadPool (support for synchronously and asynchronously waiting on arbitrary file descriptors), this would come "for free", right?

[adamsitnik]

If we move all of the async infrastructure from System.Net.Sockets into ThreadPool (support for synchronously and asynchronously waiting on arbitrary file descriptors), this would come "for free", right?

Then the synchronous implementation remains the same, but the asynchronous gains cancellation support and improved throughput (we use one global thread for polling rather than one thread per each read).