feat: add blockchain-to-notification end-to-end tests

[pimaster900]

Add end-to-end test coverage for the complete NotifyChain pipeline:
Blockchain Event → Listener → Validation → Database → Scheduler → Notification

New file:

• listener/src/tests/blockchain-to-notification-e2e.test.ts 7 describe blocks, 46 test cases covering:
  • Stage 1: RPC response validation, event payload validation, event filter matching, event name extraction
  • Stage 2: EventRegistry (in-memory store) — add, retrieve, evict, clear
  • Stage 3: NotificationDeduplicator — window-based suppression, metrics
  • Stage 4: EventSubscriber with mocked RPC server — happy path, invalid RPC, invalid payload, filter mismatch, full-contract failure
  • Stage 5: NotificationAPI + SQLite — schedule, reject past date, reject missing recipient, reject null payload
  • Stage 6: NotificationScheduler — delivery, failure, not-yet-due, batch delivery
  • Stage 7: Full pipeline — event → registry → DB → scheduler → delivery, duplicate suppression, filter rejection, delivery failure, batch

Fixes and improvements to unblock tests:

• listener/src/services/event-subscriber.ts: remove duplicate processableEvents declaration and duplicate request declaration left behind from a merge conflict; add missing backfillStartLedger private field
• listener/src/services/discord-notification.ts: fix missing closing brace on sanitizeForDiscord() left behind from a merge conflict
• listener/src/mocks/@stellar/stellar-sdk.ts: add xdr.ScVal stub (scvSymbol, scvString, scvU32) with singleton ScValType constants for switch-case equality, and scValToNative() so formatScVal works in tests without the real Stellar SDK installed

Overview

Related Issue

Closes #

Changes

Verification

# e.g.
cd dashboard && npm test
cd listener && npm run typecheck && npm test
cd contract/contracts/hello-world && cargo test

How to Test

Checklist

• Branch is up to date with main
• Tests added/updated and all pass locally
• cargo fmt --all run (if Rust changes)
• npm run lint passes (if TypeScript changes)
• Documentation updated if behavior changed

closes #796