feat(sandbox): enforce proxy-aware network routing in sandbox (#11113)

## Summary
- expand proxy env injection to cover common tool env vars
(`HTTP_PROXY`/`HTTPS_PROXY`/`ALL_PROXY`/`NO_PROXY` families +
tool-specific variants)
- harden macOS Seatbelt network policy generation to route through
inferred loopback proxy endpoints and fail closed when proxy env is
malformed
- thread proxy-aware Linux sandbox flags and add minimal bwrap netns
isolation hook for restricted non-proxy runs
- add/refresh tests for proxy env wiring, Seatbelt policy generation,
and Linux sandbox argument wiring

codex-rs/exec/tests/suite/sandbox.rs

@@ -27,6 +27,7 @@ async fn spawn_command_under_sandbox(
         sandbox_policy,
         sandbox_cwd,
         stdio_policy,
+        None,
         env,
     )
     .await
@@ -52,6 +53,7 @@ async fn spawn_command_under_sandbox(
         sandbox_cwd,
         false,
         stdio_policy,
+        None,
         env,
     )
     .await