Feature request: Implement /ready endpoint for readiness checks

**Is your feature request related to a problem? Please describe.**
Yes. Currently, arkflow-rs does expose a `/ready` endpoint for readiness checks, but currently it is just set to true after build the stream
https://github.com/arkflow-rs/arkflow/blob/92cb5d4ff5df9a6585734a608b5d590559af8937/crates/arkflow-core/src/engine/mod.rs#L244-L245. This makes it hard to integrate with orchestration systems like Kubernetes. For connectors, ready should mean that both input and output layer connections are established and healthy.

**Describe the solution you'd like**
I think that each input or output component we should define background ready check function, and in the engine, they will call these input/output ready function, report error status if any through `/ready` api
Expectation:
Add a /ready endpoint alongside the existing health checks. The readiness probe should:
- Verify input source connection is established.
- Verify output sink connection is established.
Return 200 OK when ready, and 503 Service Unavailable when not ready.

If this makes sense, I’d be happy to help contribute an implementation