Simplify ManagedWebSocket.EnsureBufferContainsAsync

[cittaz]

Summary

Cleans up ManagedWebSocket.EnsureBufferContainsAsync by removing a dead branch and a stale comment that have been left behind since #69272 replaced the manual read loop with a single Stream.ReadAtLeastAsync call. No behavior change.

Background

Before #69272 (May 2022), the method ended with a real loop:

// While we don't have enough data, read more.
while (_receiveBufferCount < minimumRequiredBytes)
{
    int numRead = await _stream.ReadAsync(_receiveBuffer.Slice(_receiveBufferCount), cancellationToken).ConfigureAwait(false);
    Debug.Assert(numRead >= 0, $"Expected non-negative bytes read, got {numRead}");
    if (numRead <= 0)
    {
        ThrowEOFUnexpected();
        break;
    }
    _receiveBufferCount += numRead;
}

That PR converted it to a single ReadAtLeastAsync call (which loops internally until enough bytes are read or EOF) and downgraded while to if, but kept both the inner predicate and the original comment.

What was wrong

if (_receiveBufferCount < minimumRequiredBytes)              // outer check
{
    if (_receiveBufferCount > 0)
    {
        _receiveBuffer.Span.Slice(_receiveBufferOffset, _receiveBufferCount).CopyTo(_receiveBuffer.Span);
    }
    _receiveBufferOffset = 0;

    // While we don't have enough data, read more.
    if (_receiveBufferCount < minimumRequiredBytes)          // dead: same condition, count unchanged
    {
        ...
    }
}

• Dead branch. Between the outer check and the inner check, only CopyTo (which shifts data within the buffer) and _receiveBufferOffset = 0 execute. Neither mutates _receiveBufferCount, so the inner predicate is provably the same as the outer one. The body always runs.
• Stale comment. // While we don't have enough data, read more. describes the original while loop. The current code performs a single ReadAtLeastAsync call (the loop is inside ReadAtLeastAsync itself), so "while" no longer matches.

Change

Remove the inner if, drop the misleading comment, and dedent the body. Behavior is unchanged.

[MihaZupan]

Thanks

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