dotnet: bound a channel's receive queue by bytes, not by 32 items

[Sniperlyf3]

Problem

A healthy SSH session could fail with channel data is not being consumed fast enough.

• Cause: AgentChannelDataPump queued at most 32 items per channel, whatever their size. A burst of small PTY output frames, such as a prompt redraw or a command that prints line by line, filled the queue before the consumer caught up.
• Where it showed up: dotnet-android-e2e on ci: run the Go agent E2E suite on Windows, and fail if it skips #60. The Android probe's SSH session was faulted on a commit whose sibling run passed the same job.
• What the bound is for: capping memory held for a consumer that has stopped. At 32 items, that was a few hundred bytes for small frames but 1 MiB for 32 KiB frames. The limit guarded the wrong quantity.

Fix

• Byte limit: each channel's queue holds at most 4 MiB of payload (MaxQueuedBytes).
• Item cap: at most 16384 items (MaxQueuedItems), so a flood of empty frames can't grow the queue's own overhead.
• Release on delivery: bytes are released when an item is delivered, not when it is dequeued, because an item handed to a stalled sink is still in memory.
• Oversized frames: an empty queue always accepts the next item, so a single frame larger than the limit is never mistaken for backpressure.
• Unchanged: a channel whose consumer really has stopped is still faulted without blocking the shared read loop (N1).

Verification

• Full suite: dotnet test dotnet/Meowshell.sln passes, 227/227.

• New tests:

  • 4096 three-byte frames into a stalled sink do not fault the channel.
  • A frame larger than the limit is accepted into an empty queue.
  • A consumer that keeps up receives 3× the limit in total without faulting.

• Mutation checks (each rule broken on purpose; a test failed every time):

  Mutation	Test that failed
  Item cap set back to 32	burst test
  Empty-queue exemption removed	oversize test
  Byte limit removed	stall test
  Bytes never released	running-total test

• Existing tests: the two stall tests and ReceiveBackpressureRequestsRemoteChannelCleanupExactlyOnce now overflow the queue by bytes, using 32 KiB frames (the agent's read size), instead of by item count.

🤖 Generated with Claude Code

https://claude.ai/code/session_013WM5HyhwRLKBDyHR6qitkk

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Generated by Claude Code