Add Read(Span<byte>) override to MultipartReaderStream

[SimonCropp]

Add Read(Span) override to MultipartReaderStream

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• unit or integration tests already exist
• You've included inline docs for your change, where applicable.

Description

Move the synchronous read implementation onto Read(Span) and have the array overload delegate to it, so a caller reading a section into a span does not pay the base Stream fallback's rented array and copy. Also add the throwing Write(ReadOnlySpan) to match the other write overrides.

[dotnet-policy-service]

Thanks for your PR, @SimonCropp. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Copilot]

🟡 Changes recommended

The new span-based sync read path calls into BufferedReadStream.Read(Span<byte>) fallback (rent+copy) and—because Read(byte[],...) now delegates—appears to introduce a performance regression for all sync reads.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates MultipartReaderStream’s synchronous read/write surface to add a Read(Span<byte>) override (and have the byte[] overload delegate to it), plus adds a throwing Write(ReadOnlySpan<byte>) override to match the existing write behavior.

Changes:

• Added Write(ReadOnlySpan<byte>) override that throws NotSupportedException.
• Routed Read(byte[], int, int) to Read(Span<byte>) and implemented the core sync read logic on the span overload.
• Updated internal sync read slices to use span-based reads.

File summaries

File	Description
src/Http/WebUtilities/src/MultipartReaderStream.cs	Moves sync read implementation to Read(Span<byte>) and adds Write(ReadOnlySpan<byte>) throw override.

Review details

Suppressed comments (1)

src/Http/WebUtilities/src/MultipartReaderStream.cs:176

• The new synchronous Read(Span<byte>) override isn’t exercised by existing multipart tests (e.g., MultipartReaderTests uses CopyToAsync/ReadAsync paths). Since this change affects the sync read implementation and is intended to improve span-based reads, please add a unit test that reads section.Body via Span<byte> (and ideally also via the byte[] overload) to validate correct data/boundary behavior and guard against regressions.

    public override int Read(byte[] buffer, int offset, int count)
        => Read(buffer.AsSpan(offset, count));

    public override int Read(Span<byte> buffer)
    {
        if (_finished)

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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