Align Windows sandbox temp permissions with the child environment (#51512)

## Why

Windows temp grants could fall back to host `TEMP`/`TMP` values and bypass read-only or denied subpaths. Sandbox permissions must match the environment passed to the child process.

## What changed

- Resolve `:tmpdir` from absolute `TEMP`/`TMP` paths in the completed workload environment, preserving access modes and missing-path rules without falling back to host values.
- Deduplicate Windows environment keys case-insensitively in a deterministic order so permission resolution and the child environment select the same value.
- Use the resolved temp policy for elevated sandbox overrides across core execution, exec-server, and direct spawning.
- Launch direct-spawn sandbox wrappers with the supplied Codex executable, materializing an inner helper only when it is that executable.

## Testing

Add Windows regression coverage for temp-only writes, read-only and denied carveouts, mixed-case and duplicate environment keys, unavailable temp paths, and empty or missing write-root overrides. Exercise core execution and direct spawning, and extend exec-server RPC coverage to the elevated backend with temp values set through the environment policy.

GitOrigin-RevId: 16b8bd4702cc434fd743a08c75a3e4515780bff3

codex-rs/exec-server/src/process_sandbox.rs

@@ -310,6 +310,7 @@ pub(crate) async fn prepare_exec_request_with_telemetry(
                 &permissions,
                 &native_sandbox_policy_cwd,
                 use_elevated,
+                &request.env,
             )
         } else {
             resolve_windows_restricted_token_filesystem_overrides(