core: stop threading SandboxPolicy through exec (#25700)

## Why

#25450 attempts a broad `SandboxPolicy` removal across several unrelated
surfaces, which makes it hard to review and still leaves new helper code
moving legacy policies around. This PR is a narrower alternative:
migrate only the exec-side Windows sandbox plumbing so the review can
focus on one production path and one compatibility boundary.

The goal is to stop threading `SandboxPolicy` through exec code without
expanding the migration into app-server, protocol, telemetry, config, or
session behavior.

## What changed

- Removed `ExecRequest::compatibility_sandbox_policy()`.
- Changed the Windows restricted-token and elevated filesystem override
helpers to accept `PermissionProfile` plus the split filesystem/network
policies instead of a `SandboxPolicy`.
- Kept the remaining legacy projection local to the writable-root
comparison that still needs to compare split policy behavior against the
legacy Windows backend model.
- Rejected restricted split filesystem policies that still grant
full-disk writes before using the Windows restricted-token backend,
preserving the previous clear-failure behavior for profiles that project
to `ExternalSandbox`.
- Updated the Windows sandbox override tests to exercise the new call
shape and cover the full-write split-profile regression.

## Verification

- `just test -p codex-core windows_restricted_token`
- `just test -p codex-core windows_elevated`

codex-rs/core/src/codex_thread.rs

@@ -76,11 +76,8 @@ pub struct ThreadConfigSnapshot {
 
 impl ThreadConfigSnapshot {
     pub fn sandbox_policy(&self) -> SandboxPolicy {
-        let file_system_sandbox_policy = self.permission_profile.file_system_sandbox_policy();
         codex_sandboxing::compatibility_sandbox_policy_for_permission_profile(
             &self.permission_profile,
-            &file_system_sandbox_policy,
-            self.permission_profile.network_sandbox_policy(),
             self.cwd.as_path(),
         )
     }