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/sandboxing/src/manager.rs

@@ -290,19 +290,18 @@ impl SandboxManager {
 
 pub fn compatibility_sandbox_policy_for_permission_profile(
     permissions: &PermissionProfile,
-    file_system_policy: &FileSystemSandboxPolicy,
-    network_policy: NetworkSandboxPolicy,
     cwd: &Path,
 ) -> SandboxPolicy {
     permissions
         .to_legacy_sandbox_policy(cwd)
         .unwrap_or_else(|_| {
+            let (file_system_policy, network_policy) = permissions.to_runtime_permissions();
             compatibility_workspace_write_policy(file_system_policy, network_policy, cwd)
         })
 }
 
 fn compatibility_workspace_write_policy(
-    file_system_policy: &FileSystemSandboxPolicy,
+    file_system_policy: FileSystemSandboxPolicy,
     network_policy: NetworkSandboxPolicy,
     cwd: &Path,
 ) -> SandboxPolicy {