Constrain Windows sandbox requirements (#23766)

# Why

Managed requirements can already constrain sandbox policy choices, but
Windows sandbox implementation selection was still resolved
independently from those requirements. That left the TUI able to
continue through the unelevated fallback even when an organization wants
to require the elevated Windows sandbox implementation.

# What

- Add `[windows].allowed_sandbox_implementations` requirements support
for the Windows `elevated` and `unelevated` implementations.
- Apply that allowlist during core config resolution so disallowed
configured or feature-selected Windows sandbox implementations fall back
to an allowed implementation with the existing requirements warning
path.
- Reuse the existing TUI Windows setup prompts to block disallowed
unelevated continuation, keep required elevated setup in front of the
user, and refuse to persist a TUI-selected Windows sandbox mode that
requirements disallow.

# Semantics

| Allowed | Selected | Effective |
| --- | --- | --- |
| `["elevated"]` | `unelevated` / unset | `elevated` |
| `["unelevated"]` | `elevated` / unset | `unelevated` |
| `["elevated", "unelevated"]` | `elevated` | `elevated` |
| `["elevated", "unelevated"]` | `unelevated` | `unelevated` |
| `["elevated", "unelevated"]` | unset | `elevated` |

Availability is handled by interactive setup surfaces after allowlist
resolution. If the effective elevated implementation is not ready,
elevated-only requirements block on setup. When unelevated is also
allowed, the UI may offer the existing unelevated fallback.

## TUI Screens

If elevated setup is not already complete:
```
  Your organization requires the default Codex agent sandbox to continue. Set it up to protect your files and control
  network access.
  Learn more <https://developers.openai.com/codex/windows>

› 1. Set up default sandbox (requires Administrator permissions)
  2. Quit
```

If admin setup fails under `["elevated"]`:
```
  Couldn't set up your sandbox with Administrator permissions

  Your organization requires the default sandbox before Codex can continue.
  Learn more <https://developers.openai.com/codex/windows>

› 1. Try setting up admin sandbox again
  2. Quit
```

# Next Steps


- extend the requirements/readout surface, such as
`configRequirements/read`, so clients can inspect the loaded
`[windows].allowed_sandbox_implementations` requirement instead of
inferring it from Windows setup state
- consider extending `windowsSandbox/readiness` as well
- update the App startup guide, setup flow, and banner surfaces so an
elevated-only requirement omits any continue-unelevated escape hatch and
blocks startup until a permitted implementation is ready;
- preserve the existing unelevated fallback path when requirements allow
it, including the `["unelevated"]` case where elevated is disallowed