Add minimal exec-server websocket reconnect

[starr-openai]

Summary

• add minimal websocket-only exec-server client reconnect for later new remote ops
• cache terminal resume failures and keep stdio one-shot
• add Docker-backed remote env regression that drops the live exec-server websocket and verifies the next remote filesystem op reconnects

Validation

• bazel test --bes_backend= --bes_results_url= //codex-rs/exec-server:exec-server-unit-tests
• bazel build --bes_backend= --bes_results_url= //codex-rs/exec-server:exec-server
• bazel build --bes_backend= --bes_results_url= //codex-rs/core:core-all-test
• source scripts/test-remote-env.sh; cd codex-rs; cargo test -p codex-core --test all remote_test_env_reconnects_after_exec_server_websocket_disconnect -- --nocapture

[chatgpt-codex-connector]

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[starr-openai]

Resubmitted as #23562 from a full-ci branch name so rust-ci-full runs.