Enforce Async-only methods in StreamConformanceTests

[rzikm]

For stream which wrap another stream (SslStream, compression streams, crypto streams, ....), we can test if async methods are calling only async methods. This should root out cases where we may be mistakenly doing blocking IO calls as part of async stack.

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[Copilot]

Pull request overview

This PR enforces async-only methods in StreamConformanceTests to ensure that async operations in wrapper streams (SslStream, compression streams, crypto streams) don't perform blocking IO calls. The changes introduce an AsyncOnlyStream wrapper that throws when synchronous methods are called, and update tests to use async operations when testing async code paths.

Key changes:

• Introduced AsyncOnlyStream wrapper to enforce async-only operations during testing
• Updated test methods to use async disposal patterns and wrap streams appropriately based on async mode
• Converted synchronous IO operations to async in SslStream tests to avoid blocking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Net.Security/tests/FunctionalTests/SslStreamConformanceTests.cs	Converted sync Read/Write calls to async ReadAsync/WriteAsync to avoid blocking during TLS 1.3 handshake flush
src/libraries/Common/tests/StreamConformanceTests/System/IO/StreamConformanceTests.cs	Added AsyncOnlyStream wrapper class, CreateWrappedStreamsAsync helper, IsAsync helper, updated test disposal patterns to use async, and added IAsyncDisposable to StreamPair

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