Add a new endpoint that allows us to test multi-turn behavior.
Tested with running:
```
RUST_LOG=codex_app_server=debug CODEX_BIN=target/debug/codex \
cargo run -p codex-app-server-test-client -- \
send-follow-up-v2 "hello" "and now a follow-up question"
```
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| .. | ||
| main.rs | ||