feat: extend MultiTenantClient with LRU, TTL, health checks, and pool…

[Eromosele0110]

… stats

• Rewrote MultiTenantClient to support a full connection pool
• Constructor now accepts PoolOptions: maxClients, ttlMs, healthCheckIntervalMs
• LRU eviction: Map insertion-order used for O(1) LRU; re-inserted on every hit
• TTL eviction: checked on each getClient() access, expired entries recreated
• Background health checks: setInterval pings each client's RPC via SorobanRpc.Server; unhealthy clients (getLatestLedger throws) are evicted, failures counted separately
• pool.stats() returns PoolStats: { size, hits, misses, evictions, healthCheckFailures }
• pool.destroy() clears the interval timer and evicts all clients
• getClient() accepts optional config override as second argument
• Exported PoolOptions and PoolStats from src/index.ts
• 25 tests covering: LRU order, maxClients boundary, TTL expiry, independent TTLs, health-check eviction, mixed-pool sweep, destroy() timer stop, stats accuracy
• Fixed pre-existing bugs on main: VersionInfo missing closing brace in types.ts, duplicate LedgerAdapter export in index.ts, version.ts string|string[] split, client.ts missing InvoiceLifecycleHooks/_hooks, client.test.ts await-in-non-async and wrong error count assertions
• close Implement a multi-tenant client pool with TTL-based eviction and health checks #423