Limit Bazel integration test threads on macOS (#39609)

## What changed

- Add a `test_threads` option to `codex_rust_crate` that sets
  `RUST_TEST_THREADS` for sharded integration tests when configured.
- Run the `core` and `app-server` Bazel integration tests with one Rust test
  thread on macOS, leaving the thread count unchanged on other platforms.

GitOrigin-RevId: 118147413eb141f557d9ee82858860617d61acce

codex-rs/app-server/BUILD.bazel

@@ -24,4 +24,8 @@ codex_rust_crate(
         "app-server-unit-tests": 8,
     },
     test_tags = ["no-sandbox"],
+    test_threads = select({
+        "@platforms//os:macos": 1,
+        "//conditions:default": 0,
+    }),
 )