fix: respect Retry-After header on 429 responses in facilitator client

[natsukingly]

Summary

The HTTP facilitator client ignores the Retry-After header on 429 (Too Many Requests) responses, using fixed exponential backoff instead. This can cause unnecessary delays or premature retries.

Changes

• Parse Retry-After header value (integer seconds) on 429 responses
• Use the server-specified delay when available
• Fall back to existing exponential backoff when Retry-After is absent or unparseable

Behavior

429 with Retry-After: 5  →  wait 5000ms
429 with Retry-After: 0  →  fall back to exponential backoff
429 without header       →  fall back to exponential backoff (unchanged)

Test plan

• Full monorepo build passes
• Verify retry behavior with mocked 429 responses