fix: use exec end output as display fallback

[zerone0x]

Summary

• Use ExecCommandEndEvent formatted_output/aggregated_output as the non-terminal display fallback when no output deltas were accumulated.
• Keep existing delta accumulation as the first choice to avoid reintroducing the O(N²) output behavior fixed for exec_command_output_delta resends entire accumulated output on every chunk, causing O(n²) memory growth and process crash on large command output #225.
• Extend the parallel exec regression test to assert completed tool-call updates include end-event output.

Fixes #290

Testing

• cargo fmt --check
• cargo test thread::tests::test_parallel_exec_commands (blocked in this container: initially no cargo; after installing rustup minimal, build failed because no system linker/libc are available: linker cc not found, then rust-lld: unable to find library -lc and related system libraries)

[benbrandt]

Hi thanks for taking the time to dig in!
Development is moving to agentclientprotocol/codex-acp. The new adapter is built on the new Codex App Server, and we are pooling implementation and maintenance work across teams there. If this is still needed, please open a PR for that one. Thanks!