Expand ~ in MDM workspace write roots (#15351)

## Summary
- Reuse the existing config path resolver for the macOS MDM managed
preferences layer so `writable_roots = ["~/code"]` expands the same way
as file-backed config
- keep the change scoped to the MDM branch in `config_loader`; the
current net diff is only `config_loader/mod.rs` plus focused regression
tests in `config_loader/tests.rs` and `config/service_tests.rs`
- research note: `resolve_relative_paths_in_config_toml(...)` is already
used in several existing configuration paths, including [CLI
overrides](https://github.com/openai/codex/blob/74fda242d3651f0a43ec8657bdbc7bde426dce0e/codex-rs/core/src/config_loader/mod.rs#L152-L163),
[file-backed managed
config](https://github.com/openai/codex/blob/74fda242d3651f0a43ec8657bdbc7bde426dce0e/codex-rs/core/src/config_loader/mod.rs#L274-L285),
[normal config-file
loading](https://github.com/openai/codex/blob/74fda242d3651f0a43ec8657bdbc7bde426dce0e/codex-rs/core/src/config_loader/mod.rs#L311-L331),
[project `.codex/config.toml`
loading](https://github.com/openai/codex/blob/74fda242d3651f0a43ec8657bdbc7bde426dce0e/codex-rs/core/src/config_loader/mod.rs#L863-L865),
and [role config
loading](https://github.com/openai/codex/blob/74fda242d3651f0a43ec8657bdbc7bde426dce0e/codex-rs/core/src/agent/role.rs#L105-L109)

## Validation
- `cargo fmt --all --check`
- `cargo test -p codex-core
managed_preferences_expand_home_directory_in_workspace_write_roots --
--nocapture`
- `cargo test -p codex-core
write_value_succeeds_when_managed_preferences_expand_home_directory_paths
-- --nocapture`

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>
Co-authored-by: Michael Bolin <bolinfest@gmail.com>