fix: keep remote exec alive after stdin EOF

[alongubkin]

Problem

For non-interactive remote exec commands, local stdin can reach EOF immediately. The stdin forwarding task then drops the last input sender, causing the WebSocket writer to close the session before a slower remote command reports its exit status.

Fix

Keep one input sender owned by the attach session. Stdin EOF now means only that no more input will be sent; the WebSocket remains open until the remote exit/error frame, timeout, or cancellation.

The regression test uses a real local WebSocket peer: stdin reaches EOF first, the peer sends a delayed exit frame, and the attach call must still succeed.

Validation

• cargo test -p alien-cli commands::debug::tests
• git diff --check

[greptile-apps]

Greptile Summary

This PR keeps the remote-exec input channel open after local stdin reaches EOF, allowing the read side to wait for the remote command’s terminal frame.

• Retains a session-owned input sender until exit, error, timeout, or cancellation.
• Adds a loopback WebSocket regression test that delays the remote exit frame until after stdin EOF.

Confidence Score: 5/5

The PR appears safe to merge, with the attach lifecycle correctly preserving the WebSocket until a terminal remote outcome is received.

All attach termination paths explicitly abort the writer before returning, so retaining the sender fixes premature EOF shutdown without introducing a reachable hang or cleanup failure.

Important Files Changed

Filename	Overview
crates/alien-cli/src/commands/debug.rs	Retains the input sender for the attach-session lifetime and adds a realistic delayed-exit regression test; no actionable defect was found.

Sequence Diagram

sequenceDiagram
    participant Stdin as Local stdin
    participant Attach as Attach session
    participant Writer as WebSocket writer
    participant Remote as Remote peer
    Stdin-->>Attach: EOF
    Note over Attach,Writer: Session-owned sender keeps input channel open
    Remote-->>Attach: Delayed exit/error frame
    Attach->>Writer: Abort writer
    Attach-->>Attach: Return remote result

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