fix(api): standardize response envelopes and resolve API test failures (#385)

[Abdullahi-Code9]

Closes #385

Summary

This PR standardizes the API response formats across all endpoints to resolve payload wrapping inconsistencies and fix cascading test failures in the listener service (#385).

Key Changes

1. API Envelope Standardization:

   • Updated GET /health in src/api/events-server.ts to return direct, unwrapped JSON objects required by health contract tests.
   • Refactored notifications-history and templates-api handlers to ensure consistent payload structures using sendJson / standardized response helpers.

2. Database & Schema Fixes:

   • Removed duplicate column declarations (next_retry_at) from schema.sql CREATE TABLE statements and handled migration safety in database.ts.
   • Fixed SQL parameter count mismatches (11 params bound to 10 columns) in scheduled-notification-repository.ts.
   • Resolved old notification_templates schema conflicts.

3. Compiler & Test Environment Fixes:

   • Fixed duplicate method implementation (setTtlMs) in src/store/event-registry.ts.
   • Added mock modules for @stellar/stellar-sdk and node-cache to ensure consistent unit test isolation.
   • Adjusted timer leak issues (.unref()) allowing Jest workers to exit cleanly.

Verification

All 78 tests across the 6 API test suites under src/api/ are now passing 100% green:

• events-server.health.test.ts (12 tests)
• events-server.test.ts
• notifications-history.test.ts (9 tests)
• templates-api.test.ts
• rate-limiter.test.ts
• execution-metrics-api.test.ts (5 tests)

Test Command Executed:
npm test -- src/api/

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