fix: ensure cwd for conversation and sandbox are separate concerns (#3874)

Previous to this PR, both of these functions take a single `cwd`:


71038381aa/codex-rs/core/src/seatbelt.rs (L19-L25)


71038381aa/codex-rs/core/src/landlock.rs (L16-L23)

whereas `cwd` and `sandbox_cwd` should be set independently (fixed in
this PR).

Added `sandbox_distinguishes_command_and_policy_cwds()` to
`codex-rs/exec/tests/suite/sandbox.rs` to verify this.

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

@@ -171,6 +171,8 @@ async fn python_getpwuid_works_under_seatbelt() {
 
     // ReadOnly is sufficient here since we are only exercising user lookup.
     let policy = SandboxPolicy::ReadOnly;
+    let command_cwd = std::env::current_dir().expect("getcwd");
+    let sandbox_cwd = command_cwd.clone();
 
     let mut child = spawn_command_under_seatbelt(
         vec![
@@ -179,8 +181,9 @@ async fn python_getpwuid_works_under_seatbelt() {
             // Print the passwd struct; success implies lookup worked.
             "import pwd, os; print(pwd.getpwuid(os.getuid()))".to_string(),
         ],
+        command_cwd,
         &policy,
-        std::env::current_dir().expect("should be able to get current dir"),
+        sandbox_cwd.as_path(),
         StdioPolicy::RedirectForShellTool,
         HashMap::new(),
     )
@@ -216,13 +219,16 @@ fn create_test_scenario(tmp: &TempDir) -> TestScenario {
 /// Note that `path` must be absolute.
 async fn touch(path: &Path, policy: &SandboxPolicy) -> bool {
     assert!(path.is_absolute(), "Path must be absolute: {path:?}");
+    let command_cwd = std::env::current_dir().expect("getcwd");
+    let sandbox_cwd = command_cwd.clone();
     let mut child = spawn_command_under_seatbelt(
         vec![
             "/usr/bin/touch".to_string(),
             path.to_string_lossy().to_string(),
         ],
+        command_cwd,
         policy,
-        std::env::current_dir().expect("should be able to get current dir"),
+        sandbox_cwd.as_path(),
         StdioPolicy::RedirectForShellTool,
         HashMap::new(),
     )