fix(relay): cancel tunnel opens on parent shutdown

[lawrencecchen]

Summary

• race tunnel frame handling, including open, against parent and connection cancellation
• close the tunnel promptly when provider open work does not return
• add a bounded regression test with a deliberately suspended PTY provider

Verification

• rustfmt --edition 2024 --check
• git diff --check
• no local Cargo tests or builds run per task constraints

This is a separate follow-up for #11401 and does not modify #11401, #11505, or #11526.

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Summary by cubic

Completes the #11401 follow-up: tunnel opens now cancel when the parent shuts down, so relay connections close promptly instead of waiting out the open timeout.

• Races every tunnel frame, including OPEN, against parent cancellation and connection close.
• Adds a bounded regression test with a suspended PTY provider.

^{Written for commit 2b360c8. Summary will update on new commits.}

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Project	Deployment	Actions	Updated
cmux166	Ready	Preview	Sep 2, 2026 9:18pm UTC
cmux41	Ready	Preview	Sep 2, 2026 9:18pm UTC

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📥 Commits

Reviewing files that changed from the base of the PR and between 0247f51 and 2b360c8.

📒 Files selected for processing (1)

• cmux-tui/crates/chatmux-relay/src/tunnel_terminal.rs

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[teamleaderleo]

Superseded by #11155, which carries this cancellation coverage together with the follow-on resource cleanup.