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/unified_exec/process_manager.rs

@@ -1246,6 +1246,7 @@ impl UnifiedExecProcessManager {
                 command: &request.command,
                 approval_policy: turn.approval_policy(),
                 permission_profile: request.turn_environment.permission_profile().clone(),
+                environment_policy: request.turn_environment.config().exec_policy.as_ref(),
                 windows_sandbox_level: turn.windows_sandbox_level,
                 sandbox_permissions: if request.additional_permissions_preapproved {
                     crate::sandboxing::SandboxPermissions::UseDefault