Implement a multi-tenant client pool with TTL-based eviction and health checks

[Kingsman-99]

Overview

Extend MultiTenantClient to support a true connection pool: each tenant's client is kept alive up to a TTL, health-checked periodically, and evicted on failure or expiry. This is critical for server-side usage (e.g. a Next.js API route serving many tenants).

Requirements

• MultiTenantClient({ maxClients: number, ttlMs: number, healthCheckIntervalMs: number })
• getClient(tenantId: string, config?: StellarSplitClientConfig): StellarSplitClient — returns existing or creates new client
• LRU eviction when maxClients is reached
• TTL-based eviction: clients older than ttlMs are evicted on next access
• Background health check: pings each client's RPC every healthCheckIntervalMs; evicts unhealthy clients
• pool.stats(): PoolStats returning { size, hits, misses, evictions, healthCheckFailures }
• Write tests covering LRU eviction, TTL expiry, health check eviction, and pool stats accuracy

Acceptance Criteria

• LRU eviction working under maxClients limit
• TTL eviction working correctly
• Health check evicts unhealthy clients
• pool.stats() returns accurate metrics
• npm test passes with zero failures
• TypeScript compiles with zero errors
• ESLint passes with zero errors

Definition of Done

All CI checks must pass before the PR is reviewed.

[nurudeenheroic]

I've gone through the issue and am ready to get started.

[Eromosele0110]

let me work on this

[grantfox-oss]

🦊 GrantFox — @Eromosele0110 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #423)
2. Your PR will be reviewed by the Stellar-split maintainers

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