[SDK · Network & Horizon] Implement rate limit aware request queue

[amina69]

Epic: Network & Horizon
Labels: network, infra, sdk
Size: M (~2-3 days)

Description

Limit outgoing Horizon requests to avoid hitting the rate limit.

Tasks

• Default: max 10 concurrent requests, 100 req/s
• Use p-queue or simple semaphore implementation
• On 429: pause all requests, wait Retry-After, resume
• Configurable via HORIZON_RATE_LIMIT_RPS env var
• Emit warning log when queue depth exceeds 20
• Unit test: concurrent requests limited, 429 pauses queue