Fix garbled PF_ConsoleLogs.txt when collecting Linux container logs

[Copilot]

Docker's GetContainerLogsAsync returns a multiplexed stream with 8-byte binary headers per frame (stream type, padding, big-endian uint32 payload size). The existing code reads this raw stream with StreamReader.ReadLine(), so those binary headers appear as �������� garbage in the log file.

Changes

• DockerContainerEngine.CollectLogs: Replace StreamReader.ReadLine() with DemuxDockerStream, which parses the Docker multiplexed stream frame-by-frame, stripping headers and writing only UTF-8 payload content
• DockerContainerEngine.DemuxDockerStream (new, internal static): Reads 8-byte header → extracts payload size → reads payload → invokes write callback. Loops until stream is exhausted.
• DockerContainerEngine.ReadExactAsync (new, private static): Ensures exact byte counts are read from the stream, handling partial reads.
• Unit tests: 4 tests covering single frame, multiple frames (stdout+stderr), empty stream, and zero-length payload frames.

Docker multiplexed stream format

┌──────────┬───────────┬────────────────┬─────────────────────┐
│ byte 0   │ bytes 1-3 │ bytes 4-7      │ N bytes             │
│ stream   │ padding   │ payload size   │ payload             │
│ (1=out,  │ (zeros)   │ (big-endian    │ (actual log text)   │
│  2=err)  │           │  uint32 = N)   │                     │
└──────────┴───────────┴────────────────┴─────────────────────┘

Also removed an unused StringBuilder allocation in the same code path.