Add Write(ReadOnlySpan<byte>) override to FileBufferingWriteStream

[SimonCropp]

Add Write(ReadOnlySpan) override to FileBufferingWriteStream

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• unit or integration tests already exist
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Description

Span-based writers such as System.Text.Json previously went through the base Stream fallback, which rents a pooled array and copies before the array overload copies again into the paged buffer. Move the write implementation onto the span overload, have the array overload delegate to it, and give PagedByteBuffer a ReadOnlySpan Add that the ReadOnlyMemory overload forwards to.

[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]

🟢 Approval recommended

The changes are small, behavior-preserving, covered indirectly by existing tests via the array/memory overload paths, and primarily improve performance for span-based writers.

Pull request overview

This PR improves FileBufferingWriteStream write performance for span-based callers by overriding Stream.Write(ReadOnlySpan<byte>), so span writers (e.g., System.Text.Json) avoid the base Stream fallback path that rents/copies via pooled arrays.

Changes:

• Add FileBufferingWriteStream.Write(ReadOnlySpan<byte>) and route the byte[] overload to it.
• Add PagedByteBuffer.Add(ReadOnlySpan<byte>) and have the ReadOnlyMemory<byte> overload delegate to it.
• Update PublicAPI.Unshipped.txt to track the new public override.

File summaries

File	Description
src/Http/WebUtilities/src/PublicAPI.Unshipped.txt	Records the added Write(ReadOnlySpan<byte>) override on FileBufferingWriteStream as unshipped public API.
src/Http/WebUtilities/src/PagedByteBuffer.cs	Adds a span-based Add overload and forwards ReadOnlyMemory<byte> writes to it to reduce copying overhead.
src/Http/WebUtilities/src/FileBufferingWriteStream.cs	Implements the span-based write override and reuses it from the array overload for improved perf and consistent behavior.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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