## Summary
- remove the Windows-specific unified-exec environment block from tool
selection
- keep `unified_exec` default-off on Windows unless the feature is
explicitly enabled
- normalize model-provided `shell_type = unified_exec` to
`shell_command` when the feature is disabled
- drop obsolete tests tied to the removed environment gate and keep the
feature-flag regression coverage
## Why
Now that the session/long-lived process backend is implemented for the
Windows sandbox, we don't need to hard disable it anymore. We will be
rolling out slowly using a feature gate.
## Impact
This allows manual Windows opt-in in CLI and app-backed flows while
preserving the existing default-off behavior for Windows users.
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Co-authored-by: canvrno-oai <kbond@openai.com>
Co-authored-by: Codex <noreply@openai.com>
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