Enforce environment-specific command policies (#38942)

## What changed

- Add an optional restrictive execution policy to `EnvironmentConfig` and merge it over the active command policy for shell execution and approval decisions.
- Reject environment policies that contain allow rules, so environment configuration can only tighten command access.
- Include the environment policy fingerprint in cached approval keys so policy changes require fresh session approvals.

## Testing

- Cover restrictions overriding saved prefix approvals, policy changes invalidating session approvals, and environment overlays remaining effective when model-specific prefix filtering is active.

GitOrigin-RevId: e94f877549ed5e0f577b8cb52f3c1872c58f029d

codex-rs/core/src/session/session.rs

@@ -144,6 +144,7 @@ impl SessionConfiguration {
             allow_login_shell: self.allow_login_shell,
             permission_profile: self.permission_profile_state.snapshot(),
             shell_environment_policy: self.shell_environment_policy.clone(),
+            exec_policy: None,
             selected_capability_roots: Vec::new(),
         }
     }