[Feature] Add contract event listener — subscribe to escrow contract events via Soroban RPC getEvents

[Dami24-hub]

Context

contracts/escrow/src/lib.rs emits initialized, released, refunded, and cancelled events. SorobanInvokeTool can invoke functions but cannot subscribe to events. A dedicated event listener would allow the agent to react to on-chain escrow state changes autonomously.

Technical Requirements

• Create backend/tools/ContractEventListener.ts with listen(contractId: string, eventTypes: string[], onEvent: (event: SorobanRpc.Api.EventResponse) => void): () => void
• Use sorobanServer.getEvents({ filters: [{ type: "contract", contractIds: [contractId] }], ... }) for polling (Soroban RPC does not have a WebSocket stream)
• Poll every config.RETRY_DELAY_MS * 2 milliseconds
• Return a stopListening function that cancels the polling interval
• Add tests mocking sorobanServer.getEvents and asserting callbacks are fired

Definition of Done

• ContractEventListener exists
• listen returns a cleanup function
• ≥ 3 tests pass

[LELOUCH-Vii]

@LELOUCH-Vii has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'd like to work on this, I've worked directly with Soroban contracts and RPC interactions before, including handling on-chain state changes (escrow, replay-protection patterns). Since Soroban RPC has no WebSocket stream, I'll implement ContractEventListener.listen() using polling via getEvents with the specified interval, return a stopListening cleanup function, and write tests mocking getEvents to verify callbacks fire correctly for each event type — meeting the ≥3 tests requirement.

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

[drips-wave]

Congratulations, @LELOUCH-Vii! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @LELOUCH-Vii: 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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