Add exec-server websocket keepalive

[starr-openai]

Summary

• send periodic websocket Ping frames from outbound exec-server websocket clients
• cover direct exec-server websocket clients plus rendezvous harness/executor websocket connections
• keep inbound axum-accepted exec-server websocket connections passive
• add focused keepalive coverage for direct and relay websocket paths

Validation

• /Users/starr/code/openai/project/dotslash-gen/bin/bazel test //codex-rs/exec-server:exec-server-unit-tests --test_filter='websocket_connection_sends_keepalive_ping|harness_connection_sends_keepalive_ping|multiplexed_executor_sends_keepalive_ping'
• /Users/starr/code/openai/project/dotslash-gen/bin/bazel test //codex-rs/exec-server:exec-server-relay-test --test_filter=multiplexed_remote_executor_routes_independent_virtual_streams