Refactor exec-server websocket pump

[starr-openai]

Why

Exec-server websocket handling had separate reader and writer tasks for the same socket. That made websocket control-frame handling asymmetric: the task reading frames could observe Ping, but the task allowed to write frames was elsewhere. This PR moves each physical websocket onto one always-running pump so the socket owner can handle application frames and websocket control frames together.

What changed

• Refactored direct exec-server websocket connections in connection.rs to use one task that owns the websocket for outbound JSON-RPC, inbound JSON-RPC, periodic keepalive pings, and Ping -> Pong replies.
• Refactored relay websocket handling in relay.rs the same way for both the harness-side logical connection and the multiplexed executor physical socket.
• Preserved the existing keepalive ownership policy: outbound direct websocket clients still send periodic pings, inbound Axum accepts only reply with pongs, and relay physical websocket endpoints keep their existing periodic pings.
• Added focused websocket pump tests for ping/pong, binary JSON-RPC, relay data, malformed relay text frames, and close/disconnect behavior.
• Reconnect behavior is intentionally left for a follow-up.

Validation

• Devbox Bazel focused unit target:
  • //codex-rs/exec-server:exec-server-unit-tests --test_filter='websocket_connection_|harness_connection_|multiplexed_executor_'

[chatgpt-codex-connector]

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