fix(ssh-gateway): preserve runner exit status requests

[cyq1017]

Description

The SSH gateway can close the client channel before late runner channel
requests, such as exit-status, finish forwarding. When runner stdout/stderr
copy returns first, deferred channel closes can race with the request forwarder
and the client may miss the remote process exit status.

This change makes runner request forwarding waitable and waits for it during
channel teardown after runner output copy returns. The wait is bounded so a
stuck request stream cannot hang teardown forever.

Documentation

• This change requires a documentation update
• I have made corresponding changes to the documentation

Related Issue(s)

This PR addresses issue #5027

Screenshots

Not applicable.

Notes

Verification:

• go test ./apps/ssh-gateway -count=1

---

Summary by cubic

Fixes apps/ssh-gateway dropping late runner exit-status by waiting for runner→client request forwarding to drain before closing the channel. Adds a bounded timeout to avoid hangs. Addresses #5027.

• Bug Fixes
  • Make runner→client request forwarding waitable and wait during teardown; bounded by channelRequestDrainTimeout.
  • Extract forwardChannelRequests and copyRunnerToClient, and add tests to ensure exit-status arrives after runner EOF and that output copy waits until requests drain.

^{Written for commit 9e6fe0a. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}