Add FtpControlStream edge case unit tests; fix Span read path heap allocation

[Copilot]

Stream.ReadByte() in the base class allocates a byte[1] on every call, defeating the goal of the Span<byte> read path. Additionally, no unit tests existed for the control stream's line-reading logic.

Changes

• Override ReadByte() with a cached readByteBuffer = new byte[1] field, so the single-byte read path no longer allocates per call:

  private readonly byte[] readByteBuffer = new byte[1];
  
  public override int ReadByte()
  {
      if (NetworkStream == null) return -1;
      var bytesRead = NetworkStream.Read(readByteBuffer, 0, 1);
      return bytesRead == 0 ? -1 : readByteBuffer[0];
  }

  Note: NetworkStream.Read(Span<byte>) is not available on the target frameworks (netstandard1.6, net452), so the cached-buffer approach is the correct level of optimization here.

• Unit tests added in FtpControlStreamTests/When_reading_control_stream.cs via a TestStream subclass that injects a MemoryStream as BaseStream, covering:

  • ReadLine: empty stream → null, LF-terminated, CRLF stripping, incomplete line (no LF) → null, null encoding throws, pre-cancelled token throws
  • ReadByte: null NetworkStream → -1, byte available, EOF → -1

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[Copilot]

Pull request overview

This PR removes per-call heap allocations from FtpControlStream’s single-byte read path and adds coverage for control-stream line parsing behavior, improving both performance and confidence in edge-case handling.

Changes:

• Override FtpControlStream.ReadByte() to use a cached byte[1] buffer instead of allocating on every call.
• Add integration tests covering ReadLine edge cases (empty stream, LF/CRLF termination, incomplete line, null encoding, pre-cancelled token) and ReadByte behavior (null stream, data available, EOF).

Reviewed changes

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

File	Description
src/CoreFtp/Infrastructure/Stream/FtpControlStream.cs	Adds cached-buffer ReadByte() override to avoid repeated allocations in the 1-byte read path used by line reading.
tests/CoreFtp.Tests.Integration/FtpControlStreamTests/When_reading_control_stream.cs	Adds tests validating ReadLine and ReadByte behavior across key edge cases.

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