feat: implement simulation cache and integrate with poller module

[Dubemtopsite]

Closes #252

Summary

Implement a short-lived cache for task eligibility simulation results to avoid redundant RPC calls when polling for due tasks.

Changes

New

• src/simulationCache.js: TTL-based cache with configurable expiration (default 30s), max size limits, and hit/miss statistics

Modified

• src/poller.js: Integrated simulation cache in checkTask() - checks cache first before RPC call
• index.js: Cache invalidation on task execution (success/failure) to ensure fresh data after last_run changes

Config

• .env.example: Added SIMULATION_CACHE_TTL and SIMULATION_CACHE_MAX_SIZE

Why

Task eligibility checks (getTaskConfig) use simulateTransaction via RPC which is expensive. When polling multiple tasks frequently, the same task gets queried repeatedly within seconds. This cache avoids redundant RPC calls while ensuring correctness.

Cache Invalidation Rules

1. TTL expiration (default 30s) - stale results auto-expire
2. Explicit invalidation - cache invalidated when task executes (since last_run changes)
3. Manual - poller.invalidateCache(taskId)
4. Size limit - LRU eviction when at max capacity

Metrics

poller.getCacheStats()
// { size, hits, misses, hitRatePercent, ttlSeconds }
Testing
- Added __tests__/simulationCache.test.js with coverage for:
  - Basic get/set
  - TTL expiration
  - Single/bulk invalidation
  - Hit rate tracking
  - Cache size limits

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[ayomideadeniran]

pr under review, if i find any wrong implementation i will notify you.