速率限制

收到 429 请降速。遵守 Retry-After 与速率限制响应头。

响应头：`RateLimit-Limit`、`RateLimit-Remaining`、`RateLimit-Reset`，429 时还有 `Retry-After`。

• Mutating /v1 create & batch advertise limits when known
• On 429, clients must honor Retry-After
• Legacy X-RateLimit-* also present