Use the read buffer in UploadFile for the SFTP write packets

[Rob-Hague]

In SftpClient.UploadFile, a buffer is allocated to read from the given stream, and for
each read, another array is allocated for the SFTP write packet (which consists of that
data prepended with headers). This change effectively leaves space at the start of the
buffer for the headers such that it can be used to assemble the packets without that
per-packet array allocation.

There are cleaner/more general ways to do this (e.g. for all packet types, leave space
for the SSH headers as well), but this gets the most impact for about as much effort as
I can be bothered with.

[Copilot]

Pull request overview

This PR reduces allocations on the SFTP upload path by reusing the upload read buffer to assemble SSH_FXP_WRITE packets (instead of allocating a new packet-sized array per write). It does this by introducing a reusable write-request buffer that reserves space for SFTP headers up front, and by updating the SFTP send/write pipeline to use that buffer.

Changes:

• Introduces SftpWriteRequestBuffer to hold [length,type,requestId,handle,offset,dataLength,data] in a single buffer (optionally from ArrayPool<byte>).
• Updates SftpClient.InternalUploadFile and SftpSession write request sending to use the shared buffer path for SSH_FXP_WRITE.
• Updates ISftpSession.RequestWrite signature/overloads and adjusts affected unit tests and SubsystemSession.SendData calls accordingly.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/Renci.SshNet.Tests/Classes/SubsystemSession_SendData_Connected.cs	Updates mock expectations to use SendData(byte[], offset, count) overload.
test/Renci.SshNet.Tests/Classes/Sftp/SftpSessionTest_DataReceived_SingleSftpMessageInSshDataMessage.cs	Updates channel send expectations to SendData(byte[], offset, count).
test/Renci.SshNet.Tests/Classes/Sftp/SftpSessionTest_DataReceived_MultipleSftpMessagesSplitOverMultipleSshDataMessages.cs	Updates channel send expectations to SendData(byte[], offset, count).
test/Renci.SshNet.Tests/Classes/Sftp/SftpSessionTest_DataReceived_MultipleSftpMessagesInSingleSshDataMessage.cs	Updates channel send expectations to SendData(byte[], offset, count).
test/Renci.SshNet.Tests/Classes/Sftp/SftpSessionTest_Connected_RequestStatVfs.cs	Updates channel send expectations to SendData(byte[], offset, count).
test/Renci.SshNet.Tests/Classes/Sftp/SftpSessionTest_Connected_RequestRead.cs	Updates channel send expectations to SendData(byte[], offset, count).
test/Renci.SshNet.Tests/Classes/Sftp/SftpFileStreamTest.cs	Adjusts ISftpSession.RequestWrite verification to match updated signature (no completion callback parameter).
test/Renci.SshNet.Tests/Classes/Sftp/Requests/SftpWriteRequestTest.cs	Updates tests to construct SftpWriteRequest from SftpWriteRequestBuffer and validates both serialization paths.
src/Renci.SshNet/SubsystemSession.cs	Adds SendData(byte[], offset, count) overload and routes existing SendData(byte[]) through it.
src/Renci.SshNet/SftpClient.cs	Uses SftpWriteRequestBuffer (pooled) in InternalUploadFile to avoid per-packet allocations.
src/Renci.SshNet/Sftp/SftpSession.cs	Special-cases SftpWriteRequest sending to avoid allocating a byte[]; refactors write request APIs to accept SftpWriteRequestBuffer.
src/Renci.SshNet/Sftp/Requests/SftpWriteRequestBuffer.cs	New helper type encapsulating an SFTP write packet buffer with header space and optional pooling.
src/Renci.SshNet/Sftp/Requests/SftpWriteRequest.cs	Refactors write request serialization to use SftpWriteRequestBuffer and expose data/handle as spans.
src/Renci.SshNet/Sftp/ISftpSession.cs	Updates write request API: removes callback from the wait-handle overload and adds a buffer-based callback overload.