fix(video-streamer): handle channel close during header write and emit a single terminal frame

[irvingouj@Devolutions (irvingoujAtDevolution)]

Summary

Implementation-level hardening of the WebM "shadow" live-streamer (video-streamer crate). The streaming architecture is unchanged; these are correctness fixes to its termination/error handling, surfaced and verified with the crate's (ignored, XMF-gated) integration harness.

Fixes

#0 — spurious server_error on early disconnect (header phase)

A client that disconnected while webm_stream was still writing the initial WebM headers caused webm_stream to return Err. The header-write loop propagated the closed-channel error with a bare ?, while the main encode loop already treated a closed channel as a clean shutdown. The caller (devolutions-gateway/src/streaming.rs) turns that Err into a server_error sent to the client — so any shadow session closed during the brief header phase got a bogus error. The header path now goes through the same closed-channel handling and exits Ok.

This was intermittent (a scheduling race between the disconnect and the header writes); the existing client_disconnect_exits_cleanly test caught it only ~1-in-3 runs.

#5 — duplicate terminal frame on shutdown

In spawn_sending_task, both the message-handler task and the control task independently emitted a terminal frame (End/Error) on most termination paths (external shutdown, client disconnect, error), so the client could receive two terminal frames. A shared AtomicBool now arbitrates: whoever flips it first sends the single terminal frame; the other skips. The clean producer-EOF path still emits exactly one End.

#8 — consolidate closed-channel detection

The closed-channel check was duplicated as a fragile triple downcast in two places. It is now a single ChannelWriterError::is_in_chain. The classifier walks the anyhow error chain and peeks inside io::Error via get_ref() — necessary because io::Error::source() exposes the inner error's source (here, nothing), not the ChannelWriterError itself.

Tests

Two new #[ignore], XMF-gated regression tests in webm_stream_correctness.rs:

• early_disconnect_during_headers_exits_ok — disconnect during the header phase must exit Ok (red→green against this fix).
• external_shutdown_emits_single_terminal_frame — at most one terminal frame on shutdown.

Verification:

• cargo clippy -p video-streamer --tests and --lib --features perf-diagnostics: clean.
• Full webm_stream_correctness suite (9 tests) passes serialized.
• An independent code review confirmed the production changes are correct (is_in_chain reasoning, the AtomicBool gate with no zero-frame regression, and the header early-return contract were all verified).

Notes / out of scope

• The XMF-gated tests need DGATEWAY_LIB_XMF_PATH and a local WebM asset under crates/video-streamer/testing-assets/ (not committed; new crate-level .gitignore keeps it out).
• Pre-existing, untouched: --features bench fails to compile (bench_support.rs:74 calls WebmPositionedIterator::new with 1 arg vs 3). Not addressed here.
• Larger items flagged during diagnosis but intentionally left out (not reproduced as production bugs): the byte-offset iterator tracking, and making the spawn_blocking decode path interruptible on shutdown.

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