Add WebSocket Heartbeat/Ping-Pong to Detect Silent Connection Death

[maugauwi-hash]

Area: WebSocket / Real-Time
Priority: Medium
Estimated Time: 1.5 hours

Description:
shared/api-contract.md says clients "are expected to reconnect with backoff on an unexpected
drop," which requires detecting the drop in the first place. A silently dead TCP connection (e.g.,
NAT timeout without a clean close frame) can go undetected for a long time without an
application-level ping.

Tasks:

• Add a periodic ping frame (or app-level heartbeat message) from both VaultEventSocket implementations
• Trigger the existing reconnect-with-backoff path if no pong/heartbeat response arrives in time
• Add tests simulating a silent connection death (no close frame, no further messages)

[Hey-Yetunde]

@Hey-Yetunde has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi I would like to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Hey-Yetunde to this issue.

[Iceeyyou2]

@Iceeyyou2 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I'd be happy to work on this issue. I'll provide a production-ready implementation, add or update tests where needed, and ensure the solution aligns with the project's existing patterns.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Iceeyyou2 to this issue.

[drips-wave]

Congratulations, @Iceeyyou2! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @Iceeyyou2: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@Iceeyyou2's PR #357 was approved and merged by @maugauwi-hash.

🏆 @Iceeyyou2: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (80 total points). Track your full progress on GrantFox.

👏 Great work, @Iceeyyou2! Keep contributing to ethos-protocol.