Exclude MemoryStream operations from VSTHRD103

[Copilot]

VSTHRD103 recommends async alternatives for synchronous MemoryStream operations, where accessing in-memory data needs no async machinery. These APIs should be exempt by default rather than requiring per-project configuration.

• Packaged defaults: Exclude Read, Write, Flush, CopyTo, and Dispose.
• Receiver matching: Match instance-method exclusions against the receiver’s static type as well as the declaring type, covering inherited overloads without suppressing diagnostics on Stream-typed receivers.
• Coverage and documentation: Add regression coverage for overloads, conditional access, and Stream-typed calls. Document the defaults and note that CopyTo destinations may still perform blocking I/O.

static async Task<int> ReadAsync(MemoryStream stream, byte[] buffer)
{
    await Task.Yield();
    return stream.Read(buffer, 0, buffer.Length); // No VSTHRD103
}

• Fixes VSTHRD103 should not report on MemoryStream operations #1699

[azure-pipelines]

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