## Why
Concurrent tests can inherit writable descriptors for executable fixtures, causing `ETXTBSY` when those fixtures launch on Linux.
## What changed
Expose shared `write_executable` and `copy_executable` helpers from `codex_utils_cargo_bin` and adopt them in CLI, exec-server, and MCP tests. On Linux, writes and copies complete in separate processes so sibling test spawns cannot inherit the writable descriptors. Script fixtures receive mode `0o755`, and copied executables retain their source permissions.
Remove the executable-busy retry loop from the program resolver test now that its script uses the shared helper.
GitOrigin-RevId: 61624158190bb1e75c27d9eb9c87a2defc147fb7
## What changed
- Run capability discovery for roots with the same sandbox context in one sandbox helper invocation.
- Keep roots with different sandbox permissions isolated, and fall back to per-root discovery if the batched invocation fails.
## Testing
- Add coverage for single-launch batching, fallback after a sandbox launch failure, and roots with different permission profiles.
GitOrigin-RevId: a6e185a8edfd028fc3639b7c6c3cc7da342b6fe3
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