fix(ssh): treat negative keepalive replies as liveness

[skevetter]

Summary\n\n- treat both positive and negative SSH keepalive replies as proof of peer liveness\n- continue reporting genuine SSH transport errors\n- add regression coverage for unsupported keepalive requests against the Devsy SSH server\n\nFixes #1210.

Summary by CodeRabbit

• Bug Fixes
  • SSH keepalive handling now remains stable when servers accept or reject keepalive requests.
  • Genuine SSH transport failures continue to be reported.
  • Normal SSH session execution remains unaffected when keepalive requests are rejected.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 2bb891b3-c660-467c-9472-a1afe98560b0

📥 Commits

Reviewing files that changed from the base of the PR and between b291bd1 and 6fd7fe6.

📒 Files selected for processing (2)

• cmd/workspace/ssh.go
• cmd/workspace/ssh_keepalive_test.go

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📝 Walkthrough

Walkthrough

The SSH keepalive handler now ignores rejected replies while preserving transport errors. Tests cover both response types and validate behavior against the production SSH server, including a normal session.

Changes

SSH keepalive handling

Layer / File(s)	Summary
Keepalive response behavior cmd/workspace/ssh.go	checkKeepAliveResponse returns errors only when the transport request fails.
Keepalive test coverage cmd/workspace/ssh_keepalive_test.go	Unit tests cover accepted and rejected replies, wrapped transport errors, and an integration flow with the production SSH server and a normal session.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 6fd7f

SSH keepalives now recognize both accepted and rejected replies as proof of liveness while continuing to report transport failures. Coverage includes the production server and normal sessions, so the change is ready to merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: treating negative SSH keepalive replies as evidence of peer liveness.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches 💡 1

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✨ Simplify code

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• Commit simplified code in branch fabulous-dragon

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