## Why
Cleanup for #31179 exposed a core fallback bug that the TUI had been
handling locally. When a custom `.rules` file fails to parse, nonfatal
clients warn and continue, but `load_exec_policy_with_warning` replaced
the entire policy with `Policy::empty()` before managed requirements
were merged. App-server and desktop clients could therefore silently
lose required prompt and forbidden rules.
This fix is intentionally separate from #31179 so the TUI cleanup does
not depend on it.
## What changed
- Preserve the managed requirements exec policy when custom file rules
fail to parse, while returning the existing warning and discarding the
file-based policy.
- Use the same nonfatal fallback when loading network proxy policy.
- Keep parse errors fatal for strict clients such as `codex exec`.
Eric Traut
831c14fc39