Derive sandbox contexts from turn environments (#40551)

## What changed

- Add Windows sandbox mode, private-desktop behavior, and legacy Landlock selection to each `EnvironmentConfig`.
- Centralize sandbox context construction on `TurnEnvironment`, preserving environment-owned permissions, paths, and backend settings while applying additional permission grants.
- Use the environment-derived context for project instructions, capability discovery, file uploads, and filesystem-oriented tools.

## Testing

- Verify capability discovery uses the selected environment's permission profile and sandbox backends when they differ from the thread defaults.

GitOrigin-RevId: 7ae2fed90c8ada950f2e8431e571e9b2c793c291

codex-rs/windows-sandbox-rs/src/unified_exec/mod.rs

@@ -23,6 +23,8 @@ use std::path::PathBuf;
 ///
 /// Callers should parse their own input shape first, then use this request to
 /// share the elevated-vs-legacy backend selection and session launch path.
+// TODO(anp): Reconcile Windows backend and desktop copies with the supplied sandbox
+// context (TurnEnvironment::sandbox_context for turns), preserving this launch snapshot.
 pub struct WindowsSandboxSessionRequest<'a> {
     pub permission_profile: &'a PermissionProfile,
     pub workspace_roots: &'a [AbsolutePathBuf],