Optimize JsonReaderData for MemoryStream and make buffer publicly visible for StructuralJsonTypeMappings

[JoasE]

Internally managed providers pass MemoryStream to JsonReaderData. This PR makes their internal buffer accessible and optimizes JsonReaderData to leverage the internal buffer if possible. This prevents duplicate buffering for json MemoryStreams

[JoasE]

I don’t do these types of optimizations often, but I ran into this when working on json serialization for cosmos.
I think the TryGetBuffer is a must have, but also have some less substantiated ideas like calling ToArray on smaller streams and increasing initial buffer size for bigger streams. I removed them because I decided I was just guessing at that point, but if you want to take a look at them, you can see them in the Remove guesswork commit.

[Copilot]

Pull request overview

This PR optimizes JSON reading for provider-produced MemoryStream instances by allowing JsonReaderData to directly use a MemoryStream’s internal buffer (when available), avoiding duplicate buffering and reducing allocations during structural JSON materialization.

Changes:

• Updated JsonReaderData to store its input as ReadOnlyMemory<byte> and added a MemoryStream fast-path using TryGetBuffer().
• Updated SQL Server structural JSON mapping to create MemoryStream instances with a publicly visible buffer (so TryGetBuffer() succeeds).
• Updated SQLite structural JSON mapping to use a shared CreateUtf8Stream helper (also creating MemoryStream with a publicly visible buffer) and simplified the expression tree.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/EFCore/Storage/Json/JsonReaderData.cs	Adds MemoryStream buffer fast-path and shifts internal buffering to ReadOnlyMemory<byte> + optional mutable buffer.
src/EFCore.SqlServer/Storage/Internal/SqlServerStructuralJsonTypeMapping.cs	Creates UTF-8 MemoryStream with publiclyVisible: true to enable buffer reuse in JsonReaderData.
src/EFCore.Sqlite.Core/Storage/Internal/SqliteJsonTypeMapping.cs	Introduces CreateUtf8Stream helper and uses it from CustomizeDataReaderExpression, enabling buffer reuse.

[roji]

BTW @JoasE our whole JSON reading architecture here is quite inefficient: we read the JSON string, and then expose that as UTF8 data as bytes to be decoded (again!) by Utf8JsonReader etc. The real perf win here would be to have have Utf8JsonReader read directly from the network with any additional buffering/decoding, see #31216.

[JoasE]

@roji Thanks for the quick review!

BTW @JoasE our whole JSON reading architecture here is quite inefficient: we read the JSON string, and then expose that as UTF8 data as bytes to be decoded (again!) by Utf8JsonReader etc. The real perf win here would be to have have Utf8JsonReader read directly from the network with any additional buffering/decoding, see #31216.

Good point, I guess my current changes tackle some low-hanging fruit in the meantime, which should also benefit the new Cosmos deserialization work.