feat: poll Soroban RPC getEvents for contract_event subscriptions

[cokehill]

Summary

Contract events are not available over Horizon SSE. We poll the Soroban JSON-RPC
getEvents method on a configurable interval.

Acceptance criteria

• StreamManager._streamContractEvents() polls getEvents using fetch
• filter.poll_interval_ms controls interval (default 5000ms)
• filter.topics passed through to the RPC filter array
• Tracks last seen ledger to avoid duplicate deliveries
• Gracefully handles RPC errors (logs, does not crash)
• Integration test against Soroban testnet RPC (or mocked response)

Notes

Soroban RPC: https://developers.stellar.org/docs/data/rpc/api-reference/methods/getEvents
Default testnet RPC: https://soroban-testnet.stellar.org