Avoid blocking OkHttp WebSocket callbacks

[masaru87]

Summary

• replace the rendezvous receive channel with a finite buffered channel
• use non-blocking trySend from the OkHttp WebSocket callback
• close the receive flow with MimiIOException if the bounded buffer rejects a response
• verify that a response received before Flow collection returns promptly and remains available to the collector

The previous Channel() and trySendBlocking combination blocked the OkHttp listener thread until a Flow collector was ready. A response arriving between connect completion and collection could therefore stall the WebSocket callback.

Testing

• ./gradlew :engine:okhttp:test --no-daemon
• ./gradlew jvmTest --no-daemon