core: make shell behavior portable on FreeBSD (#7039)

- Use /bin/sh instead of /bin/bash on FreeBSD/OpenBSD in the process
group timeout test to avoid command-not-found failures.

- Accept /usr/local/bin/bash as a valid SHELL path to match common
FreeBSD installations.

- Switch the shell serialization duration test to /bin/sh for improved
portability across Unix platforms.

With this change, `cargo test -p codex-core --lib` runs and passes on
FreeBSD.

codex-rs/core/tests/suite/shell_serialization.rs

@@ -366,7 +366,7 @@ async fn shell_output_for_freeform_tool_records_duration(
     let test = builder.build(&server).await?;
 
     let call_id = "shell-structured";
-    let responses = shell_responses(call_id, vec!["/bin/bash", "-c", "sleep 1"], output_type)?;
+    let responses = shell_responses(call_id, vec!["/bin/sh", "-c", "sleep 1"], output_type)?;
     let mock = mount_sse_sequence(&server, responses).await;
 
     test.submit_turn_with_policy(