fix(linux-sandbox): ignore missing writable roots (#14890)

## Summary
- skip nonexistent `workspace-write` writable roots in the Linux
bubblewrap mount builder instead of aborting sandbox startup
- keep existing writable roots mounted normally so mixed Windows/WSL
configs continue to work
- add unit and Linux integration regression coverage for the
missing-root case

## Context
This addresses regression A from #14875. Regression B will be handled in
a separate PR.

The old bubblewrap integration added `ensure_mount_targets_exist` as a
preflight guard because bubblewrap bind targets must exist, and failing
early let Codex return a clearer error than a lower-level mount failure.

That policy turned out to be too strict once bubblewrap became the
default Linux sandbox: shared Windows/WSL or mixed-platform configs can
legitimately contain a well-formed writable root that does not exist on
the current machine. This PR keeps bubblewrap's existing-target
requirement, but changes Codex to skip missing writable roots instead of
treating them as fatal configuration errors.

codex-rs/linux-sandbox/tests/suite/landlock.rs

@@ -310,6 +310,32 @@ async fn test_writable_root() {
     .await;
 }
 
+#[tokio::test]
+async fn sandbox_ignores_missing_writable_roots_under_bwrap() {
+    if should_skip_bwrap_tests().await {
+        eprintln!("skipping bwrap test: bwrap sandbox prerequisites are unavailable");
+        return;
+    }
+
+    let tempdir = tempfile::tempdir().expect("tempdir");
+    let existing_root = tempdir.path().join("existing");
+    let missing_root = tempdir.path().join("missing");
+    std::fs::create_dir(&existing_root).expect("create existing root");
+
+    let output = run_cmd_result_with_writable_roots(
+        &["bash", "-lc", "printf sandbox-ok"],
+        &[existing_root, missing_root],
+        LONG_TIMEOUT_MS,
+        false,
+        true,
+    )
+    .await
+    .expect("sandboxed command should execute");
+
+    assert_eq!(output.exit_code, 0);
+    assert_eq!(output.stdout.text, "sandbox-ok");
+}
+
 #[tokio::test]
 async fn test_no_new_privs_is_enabled() {
     let output = run_cmd_output(