```text

xds: implement separate request and response draining in ExternalProcessorServerInterceptor

Update ExternalProcessorServerInterceptor to support separate request-side
and response-side draining per the updated gRFC and Envoy ext_proc proto:

- Replace the unified DRAINING stream state with directional draining
  and pass-through state (requestDraining, requestDrainComplete,
  responseDraining, responseDrainComplete, requestPassThrough, and
  passThroughMode).
- Split flow-control readiness into isRequestSidecarReady() and
  isResponseSidecarReady() so draining one direction does not block or
  prematurely unblock the other direction.
- Implement the two-way drain_complete body handshake for request and
  response streams when body processing is configured, and ignore drain
  requests after end_of_stream has already been sent for that direction.
- Close the ext_proc stream once both directions have completed draining
  (in non-observability mode), and treat premature OK termination from
  ext_proc before both directions are drained or at EOS as an error.
- Update existing server interceptor draining tests and add unit tests
  for directional draining and EOS edge cases.

TAG=agy
CONV=2c1e4760-c239-4698-810a-162bf10fccc4
```

Author: kannanjgithub