[codex] Tighten unified exec sandbox setup (#22207)

## Summary
- tighten unified exec sandbox initialization
- preserve the requested process workdir independently from sandbox
setup
- add regression coverage for the updated invariant

## Validation
- Ran `/tmp/cargo-tools/bin/just fmt`.
- Ran the targeted `codex-core` regression test successfully.
- Ran `cargo test -p codex-core`; it did not complete cleanly because
unrelated existing agent/config-loader tests failed and the run later
aborted on a stack overflow in
`tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed`.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/unified_exec/mod.rs

@@ -92,6 +92,7 @@ pub(crate) struct ExecCommandRequest {
     pub yield_time_ms: u64,
     pub max_output_tokens: Option<usize>,
     pub cwd: AbsolutePathBuf,
+    pub sandbox_cwd: AbsolutePathBuf,
     pub environment: Arc<Environment>,
     pub network: Option<NetworkProxy>,
     pub tty: bool,