feat: add WithOnRateLimit callback option

[elliothllm]

Summary

• Adds WithOnRateLimit(fn func(RateLimitEvent)) option to the client
• The callback fires on every 429 response, including retries that will succeed
• RateLimitEvent includes method, path, attempt number, max retries, and retry delay
• Passes method and path through doWithRetry to provide full context in the callback

Test plan

• Existing tests pass (go test ./...)
• Verify callback fires in miranda-webhook-dashboard with mock-inflow rate limiting