tests: avoid ambient temp sandbox roots (#22576)

## Why
Some sandboxed integration tests enabled both ambient temp roots
(`TMPDIR` and literal `/tmp`) even though they were not testing
temp-root behavior. On Linux bwrap, making `/tmp` writable causes
protected metadata mount targets such as `/tmp/.git`, `/tmp/.agents`,
and `/tmp/.codex` to be synthesized. If a run is interrupted, those
top-level markers can be left behind and contaminate later tests.

## What changed
For the incidental integration tests that do not need ambient temp-root
access, set `exclude_tmpdir_env_var` and `exclude_slash_tmp` to `true`.
Dedicated protected-metadata coverage remains in the lower-level sandbox
tests that use isolated temp roots.

## Verification
Focused remote devbox repros passed with a watcher polling `/tmp/.git`,
`/tmp/.agents`, and `/tmp/.codex`; no leaked markers were observed.

codex-rs/exec/tests/suite/sandbox.rs

@@ -183,8 +183,8 @@ async fn python_multiprocessing_lock_works_under_sandbox() {
     let policy = SandboxPolicy::WorkspaceWrite {
         writable_roots,
         network_access: false,
-        exclude_tmpdir_env_var: false,
-        exclude_slash_tmp: false,
+        exclude_tmpdir_env_var: true,
+        exclude_slash_tmp: true,
     };
 
     let python_code = r#"import multiprocessing