test: isolate exec review policy config test

[bolinfest]

Why

thread_start_params_include_review_policy_when_review_policy_is_manual_only builds a Config with a temporary CODEX_HOME, but ConfigBuilder::default() can still load host-managed configuration. On local macOS machines with enterprise-managed Codex config, that host state can leak into the test and change the resulting config, even though CI does not have the same managed config source.

This makes the test environment-dependent: it can pass in CI while failing locally for developers who have managed configuration installed.

What Changed

• Updated codex-rs/exec/src/lib_tests.rs so the test calls LoaderOverrides::without_managed_config_for_tests() through ConfigBuilder::loader_overrides(...).
• Left the rest of the test setup intact, including the temporary CODEX_HOME, temporary cwd, and explicit approvals_reviewer harness override.

Verification

cargo test -p codex-exec thread_start_params_include_review_policy_when_review_policy_is_manual_only