Fixes dotnet#82801

This PR introduces the initial API prototype for System.IO.Stream factory methods and extension methods, establishing standardized stream wrappers to convert text-based and memory types into streams. This implementation follows the validated scope from the initial API review and establishes a baseline for future benchmarking, design iteration, and API evolution.

Implemented Stream Wrappers

The following stream wrappers are implemented, each providing high correctness test coverage and conformance/complementary behavioral tests:

• ReadOnlyTextStream: Wraps a string or ReadOnlyMemory<char> as a seekable, read-only stream with on-demand encoding. Particularly efficient for slicing and non-allocating substring scenarios.
• MemoryByteStream: Wraps Memory<byte> as a seekable, writable stream with a fixed-size buffer, or ReadOnlyMemory<byte> as a seekable, read-only stream.
• ReadOnlySequenceStream: Wraps ReadOnlySequence<byte> as a seekable, read-only stream. Available via extension method in System.Memory.

API Surface

Static methods on System.IO.Stream (in System.Private.CoreLib):

public static Stream FromText(string text, Encoding? encoding = null);
public static Stream FromText(ReadOnlyMemory<char> text, Encoding? encoding = null);
public static Stream FromReadOnlyData(ReadOnlyMemory<byte> data);
public static Stream FromWritableData(Memory<byte> data);

C#14 Extension method (in System.Memory):

public static class ReadOnlySequenceStreamExtensions
{
    extension(Stream)
    {
        public static Stream FromReadOnlyData(ReadOnlySequence<byte> sequence) =>
            new ReadOnlySequenceStream(sequence);
    }
}

Design Notes

• Buffer Ownership: Stream wrappers do not own or expand their provided data/buffers.
• Consistency: Disposing a stream wrapper does not alter the underlying data/buffer.
• Usability: Writing past capacity throws; reading past the end returns zero, per .NET convention.
• Seekable: All stream implementations support seeking (CanSeek = true).

Usage Example

using System.IO;
using System.Text;

// Create a stream from a string for HTTP content
Stream stream = Stream.FromText("Hello world", Encoding.UTF8);
// Use with HttpClient, File I/O, etc.

// Create a read/write stream over a Memory buffer
Memory<byte> buffer = new byte[4096];
using Stream writableStream = Stream.FromWritableData(buffer);
// ... perform stream operations

// Create a read-only stream from ReadOnlySequence<byte>
ReadOnlySequence<byte> sequence = ...;
using Stream sequenceStream = sequence.AsStream();

Implementation Goals

• High fidelity to .NET conventions and expectations around stream ownership and buffer lifetime.
• High correctness through exhaustive test coverage for all implemented wrappers and API behaviors.
• Agility to extend/adjust API and implementation in response to future dotnet/runtime API review and benchmarking.