Use exponential buffer growth in LoopbackServer test helper

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Description

LoopbackServer.Connection grows its read buffers by a fixed +BufferSize (4000 bytes) on each resize, causing O(n²) copying for large payloads. Changed to *2 doubling in both ReadToEndAsync and ReadLineBytesAsync using Array.Resize.

ReadToEndAsync:

• Fixed the resize trigger from bytesRead == buffer.Length to offset == buffer.Length (the previous check would stop triggering after the first resize, since subsequent reads only fill the remaining buffer space)
• Replaced manual new byte[] + CopyTo with Array.Resize(ref buffer, buffer.Length * 2)
• Removed the redundant totalLength variable; the final GetString now uses offset directly

ReadLineBytesAsync:

• Replaced manual new byte[] + Array.Copy (into new buffer) with in-place Array.Copy to compact live data, followed by Array.Resize(ref _readBuffer, _readBuffer.Length * 2)
• Index resets (_readStart, _readEnd, startSearch) are now always applied unconditionally after the compact step

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Pull request overview

This PR updates the HTTP/1.1 loopback test helper used across System.Net.Http functional tests to grow two internal read buffers exponentially instead of by fixed 4 KB increments, aiming to reduce repeated copying when handling large payloads.

Changes:

• Changes Connection.ReadToEndAsync to resize its temporary buffer with * 2 growth.
• Changes Connection.ReadLineBytesAsync to resize _readBuffer with * 2 growth.
• Keeps the change scoped to the shared LoopbackServer test infrastructure used by networking tests.

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/Net/Http/LoopbackServer.cs:643

• When dataLength is 0 (for example, the previous line ended exactly at the end of _readBuffer), this branch allocates a larger array but never installs it or resets the indices. The subsequent read then uses a count of 0 and ReadLineBytesAsync treats that as EOF, so the helper can stop parsing even though more request data is still available.

                            byte[] newBuffer = new byte[_readBuffer.Length * 2];
                            int dataLength = _readEnd - _readStart;
                            if (dataLength > 0)
                            {
                                Array.Copy(_readBuffer, _readStart, newBuffer, 0, dataLength);
                                _readStart = 0;
                                _readEnd = dataLength;
                                _readBuffer = newBuffer;
                                startSearch = dataLength;

[agocke]

/ba-g infra failures: Remote machine provider issue: The remote provider was unable to process the request.