CRITICAL: Schema Cache inFlight Map race condition causes duplicate fetches

## 🔴 CRITICAL BUG - Resource Waste

**Severity:** P0 - Fix Immediately

**Location:** `src/schema-cache.ts:186-199`

**Impact:** 100 concurrent calls = 100 network fetches instead of 1

**Fix:** Add double-check pattern or AsyncLock protection

**Lines to Change:** 5