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.

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