feat: migrate to new constraint-based loading strategy (#8251)

This is a significant change to how layers of configuration are applied.
In particular, the `ConfigLayerStack` now has two important fields:

- `layers: Vec<ConfigLayerEntry>`
- `requirements: ConfigRequirements`

We merge `TomlValue`s across the layers, but they are subject to
`ConfigRequirements` before creating a `Config`.

How I would review this PR:

- start with `codex-rs/app-server-protocol/src/protocol/v2.rs` and note
the new variants added to the `ConfigLayerSource` enum:
`LegacyManagedConfigTomlFromFile` and `LegacyManagedConfigTomlFromMdm`
- note that `ConfigLayerSource` now has a `precedence()` method and
implements `PartialOrd`
- `codex-rs/core/src/config_loader/layer_io.rs` is responsible for
loading "admin" preferences from `/etc/codex/managed_config.toml` and
MDM. Because `/etc/codex/managed_config.toml` is now deprecated in favor
of `/etc/codex/requirements.toml` and `/etc/codex/config.toml`, we now
include some extra information on the `LoadedConfigLayers` returned in
`layer_io.rs`.
- `codex-rs/core/src/config_loader/mod.rs` has major changes to
`load_config_layers_state()`, which is what produces `ConfigLayerStack`.
The docstring has the new specification and describes the various layers
that will be loaded and the precedence order.
- It uses the information from `LoaderOverrides` "twice," both in the
spirit of legacy support:
- We use one instances to derive an instance of `ConfigRequirements`.
Currently, the only field in `managed_config.toml` that contributes to
`ConfigRequirements` is `approval_policy`. This PR introduces
`Constrained::allow_only()` to support this.
- We use a clone of `LoaderOverrides` to derive
`ConfigLayerSource::LegacyManagedConfigTomlFromFile` and
`ConfigLayerSource::LegacyManagedConfigTomlFromMdm` layers, as
appropriate. As before, this ends up being a "best effort" at enterprise
controls, but is enforcement is not guaranteed like it is for
`ConfigRequirements`.
- Now we only create a "user" layer if `$CODEX_HOME/config.toml` exists.
(Previously, a user layer was always created for `ConfigLayerStack`.)
- Similarly, we only add a "session flags" layer if there are CLI
overrides.
- `config_loader/state.rs` contains the updated implementation for
`ConfigLayerStack`. Note the public API is largely the same as before,
but the implementation is quite different. We leverage the fact that
`ConfigLayerSource` is now `PartialOrd` to ensure layers are in the
correct order.
- A `Config` constructed via `ConfigBuilder.build()` will use
`load_config_layers_state()` to create the `ConfigLayerStack` and use
the associated `ConfigRequirements` when constructing the `Config`
object.
- That said, a `Config` constructed via
`Config::load_from_base_config_with_overrides()` does _not_ yet use
`ConfigBuilder`, so it creates a `ConfigRequirements::default()` instead
of loading a proper `ConfigRequirements`. I will fix this in a
subsequent PR.

Then the following files are mostly test changes:

```
codex-rs/app-server/tests/suite/v2/config_rpc.rs
codex-rs/core/src/config/service.rs
codex-rs/core/src/config_loader/tests.rs
```

Again, because we do not always include "user" and "session flags"
layers when the contents are empty, `ConfigLayerStack` sometimes has
fewer layers than before (and the precedence order changed slightly),
which is the main reason integration tests changed.

codex-rs/core/src/config_loader/tests.rs

@@ -66,13 +66,24 @@ async fn returns_empty_when_all_layers_missing() {
     let layers = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
         .await
         .expect("load layers");
-    let base_table = layers.user.config.as_table().expect("base table expected");
+    assert!(
+        layers.get_user_layer().is_none(),
+        "no user layer when CODEX_HOME/config.toml does not exist"
+    );
+
+    let binding = layers.effective_config();
+    let base_table = binding.as_table().expect("base table expected");
     assert!(
         base_table.is_empty(),
         "expected empty base layer when configs missing"
     );
-    assert!(
-        layers.system.is_none(),
+    let num_system_layers = layers
+        .layers_high_to_low()
+        .iter()
+        .filter(|layer| matches!(layer.name, super::ConfigLayerSource::System { .. }))
+        .count();
+    assert_eq!(
+        num_system_layers, 0,
         "managed config layer should be absent when file missing"
     );