Tighten LimitArrayPoolWriteStream cleanup contract

[Copilot]

Make LimitArrayPoolWriteStream.Dispose unconditionally throw.
This is an internal type where we always control the lifetime. The user should never be disposing it.
See #128083 (comment)

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Tightens the cleanup contract for the internal LimitArrayPoolWriteStream used by HttpContent buffering. Previously Dispose quietly returned pooled buffers, which meant a user-supplied SerializeToStream(Async) override that disposed the passed-in stream could silently corrupt the buffered content / ArrayPool.Shared. After this PR, Dispose is reserved for "the user did something wrong" and throws, while all framework-side cleanup goes through the new internal ReturnAllPooledBuffers() directly.

Changes:

• LimitArrayPoolWriteStream.Dispose(bool) now throws InvalidOperationException (with explanatory comment); ReturnAllPooledBuffers is promoted from private to internal.
• All framework callsites (HttpContent.LoadIntoBuffer[Async], LoadIntoBufferAsyncCore, HttpContent.Dispose, HttpClient.GetStringAsyncCore / GetByteArrayAsyncCore) replace Dispose() / using with explicit ReturnAllPooledBuffers() calls in finally/catch.
• New unit test verifies that disposing the buffered stream from a user SerializeToStreamAsync override surfaces InvalidOperationException out of LoadIntoBufferAsync; StreamToStreamCopyTest cleanup is simplified to best-effort.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Net.Http/src/System/Net/Http/HttpContent.cs	Switches internal cleanup paths to ReturnAllPooledBuffers, exposes it internal, and makes Dispose throw to catch user misuse.
src/libraries/System.Net.Http/src/System/Net/Http/HttpClient.cs	Replaces using var buffer = ... patterns in GetStringAsyncCore / GetByteArrayAsyncCore with nullable locals and explicit ReturnAllPooledBuffers() in finally.
src/libraries/System.Net.Http/tests/UnitTests/HttpContentTest.cs	Adds regression test asserting InvalidOperationException when a user SerializeToStreamAsync disposes the buffered stream.
src/libraries/System.Net.Http/tests/UnitTests/StreamToStreamCopyTest.cs	Drops using on LimitArrayPoolWriteStream instances (now invalid) and inserts best-effort ReturnAllPooledBuffers() calls in the resizing test.