test(indexer): comprehensive API, SSE, and rate-limit tests

[EmmanuelOchaje]

Summary

This PR adds comprehensive automated tests for critical indexer API, Server-Sent Events (SSE), and rate-limiting features to ensure robust marketplace functionality.

Test Coverage

Issue #580: GET /wallets/:address/preferences Fallback

• Verifies endpoint returns empty object {} when no preferences exist for a user
• Tests database error handling with proper 500 status response
• Confirms correct wallet address query parameter usage
• Enables frontend to implement default preferences locally (dark theme, XLM currency, alerts disabled)

Issue #581: SSE /wallets/:address/events Event Filtering

• Validates SSE connections only broadcast events relevant to specific wallet
• Tests event filtering by actor, recipient, and data fields
• Covers wallet role matching: buyer, artist, offerer, bidder, winner, creator
• Handles royalty recipient arrays (both object and string formats)
• Tests edge cases: null/undefined data, missing recipients

Issue #582: SSE Connection Cleanup on Disconnect

• Verifies SSE connection headers are correctly set
• Tests automatic connection cleanup when client disconnects
• Validates heartbeat mechanism and resource cleanup
• Confirms sseClients set properly removes disconnected clients

Issue #583: Rate-Limit Middleware IP Blocking

• Tests standard rate limiter (100 requests/minute) blocks IP after limit exceeded
• Tests strict rate limiter (20 requests/minute) for resource-intensive endpoints
• Validates RateLimit-* headers (limit, remaining, reset)
• Confirms health check endpoint bypasses rate limiting
• Validates 429 status code with appropriate error messages
• Tests retry-after information in rate limit responses
• Verifies per-IP tracking for DDoS prevention

Test Quality

• Edge Cases: Null/undefined data, empty arrays, various recipient formats
• Error Scenarios: Database failures, connection timeouts, malformed requests
• Header Validation: Proper SSE headers, rate-limit metadata headers
• Timing: Rate limit window calculations and reset timestamps
• DDoS Prevention: Per-IP tracking and strict endpoint limits

Files Modified

• indexer/src/__tests__/api.test.ts - Added 40+ test cases for preferences and SSE
• indexer/src/__tests__/rate-limit.test.ts - Enhanced with 15+ comprehensive rate limit tests

Code Patterns Followed

• Uses Vitest framework consistent with existing test suite
• Mock setup with vi.hoisted() for module-level mocking
• Supertest for HTTP endpoint testing
• Clear test descriptions following existing conventions
• Comprehensive error handling and edge case coverage

Closes #580
Closes #581
Closes #582
Closes #583

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[Ultra-Tech-code]

Hello, kindly fix conflicts and CI failure. Thank you for your contributions.