fix(server): prevent exec relays from hanging on idle connections

[Gal-Zaidman]

Summary

Gateway ExecSandbox calls could hang indefinitely after the command finished, when a supervisor session reset mid-exec orphaned the relay channel — the exec loop blocked on channel.wait() with no liveness backstop, so callers hung until their own deadline. This adds SSH and HTTP/2 keepalives and bounds the post-exit wait so a wedged/orphaned relay fails fast instead of hanging.

Related Issue

Closes #1990

Changes

• Exec relay russh client: SSH keepalive so a wedged/orphaned relay is shed (returns an error) instead of parking on channel.wait() forever. Channel-silent execs (e.g. an agent that redirects stdout to a file) stay alive while the relay is healthy — liveness is probed via keepalive, not output-idle.
• Bound how long the gateway waits for the trailing channel close after a command reports its exit status.
• Return UNAVAILABLE when a relay closes before reporting an exit status, instead of a misleading exit code 1.
• Server-side HTTP/2 keepalive (with the required Timer) on supervisor multiplex connections, to reduce the session resets that orphan relays.
• Documented the relay-liveness backstops in architecture/gateway.md.

Testing

• mise run pre-commit — rust:format:check, cargo clippy -D warnings, and markdownlint are clean for this change (ran individually). Note: the local mise run pre-commit aborts on its python:proto step due to a missing grpc_tools dev dependency in the venv, unrelated to this change; CI runs the full suite.
• Unit tests added/updated — none; the exec loop requires a live SSH relay, so there is no isolated unit surface.
• E2E tests added/updated — N/A; reproducing the fix path needs a mid-exec supervisor-session reset, which isn't reliably stageable in an in-process e2e.

Manually validated on a Kubernetes deployment: rebuilt and deployed the gateway image, confirmed the gateway is healthy, that long channel-silent execs are not killed by the keepalive, and that the previously-observed multi-sandbox hang no longer reproduces.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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[Gal-Zaidman]

I have read the DCO document and I hereby sign the DCO.

[Gal-Zaidman]

recheck

[TaylorMutch]

/ok to test b4878be

[TaylorMutch]

@Gal-Zaidman have you been able to verify this resolves the issue in your environment?

[Gal-Zaidman]

@Gal-Zaidman have you been able to verify this resolves the issue in your environment?

Yes, currently ran a job with 80 concurrent agents each running an SWE bench task with long exec (that is how harbor works) - zero hangs.
Before the fix, even with 20 agents, more than half would have hang.