fix: perform cleaning cache block store on /blocks call

[TarikGul]

Fix OOM regression in BlocksController caching

Problem: Sidecar v19.3.1 crashes with OOM after ~20 hours due to unbounded cache growth.

Root Cause:

• Polkadot.js cache capacity set to 0, disabling LRU eviction
• Controller-level cache lacks cleanup mechanism

Fix:

1. Restore polkadot.js cache capacity from 0 to 1000
2. Add periodic cache cleanup every 1000 requests (removes 20% of entries when >80% full)
3. Comprehensive test suite to prevent regression

Impact: Eliminates production OOM crashes while maintaining cache performance benefits.

Files Changed:

• src/apiRegistry/index.ts - Restore cache capacity
• src/controllers/blocks/BlocksController.ts - Add cleanup mechanism
• src/controllers/blocks/*.spec.ts - Cache test suite
• src/test/utils/cacheTestUtils.ts - Test utilities

Testing: 135 new tests covering memory leak prevention, performance, and edge cases.