feat: change ConfigLayerName into a disjoint union rather than a simple enum (#8095)

This attempts to tighten up the types related to "config layers."
Currently, `ConfigLayerEntry` is defined as follows:


bef36f4ae7/codex-rs/core/src/config_loader/state.rs (L19-L25)

but the `source` field is a bit of a lie, as:

- for `ConfigLayerName::Mdm`, it is
`"com.openai.codex/config_toml_base64"`
- for `ConfigLayerName::SessionFlags`, it is `"--config"`
- for `ConfigLayerName::User`, it is `"config.toml"` (just the file
name, not the path to the `config.toml` on disk that was read)
- for `ConfigLayerName::System`, it seems like it is usually
`/etc/codex/managed_config.toml` in practice, though on Windows, it is
`%CODEX_HOME%/managed_config.toml`:


bef36f4ae7/codex-rs/core/src/config_loader/layer_io.rs (L84-L101)

All that is to say, in three out of the four `ConfigLayerName`, `source`
is a `PathBuf` that is not an absolute path (or even a true path).

This PR tries to uplevel things by eliminating `source` from
`ConfigLayerEntry` and turning `ConfigLayerName` into a disjoint union
named `ConfigLayerSource` that has the appropriate metadata for each
variant, favoring the use of `AbsolutePathBuf` where appropriate:

```rust
pub enum ConfigLayerSource {
    /// Managed preferences layer delivered by MDM (macOS only).
    #[serde(rename_all = "camelCase")]
    #[ts(rename_all = "camelCase")]
    Mdm { domain: String, key: String },
    /// Managed config layer from a file (usually `managed_config.toml`).
    #[serde(rename_all = "camelCase")]
    #[ts(rename_all = "camelCase")]
    System { file: AbsolutePathBuf },
    /// Session-layer overrides supplied via `-c`/`--config`.
    SessionFlags,
    /// User config layer from a file (usually `config.toml`).
    #[serde(rename_all = "camelCase")]
    #[ts(rename_all = "camelCase")]
    User { file: AbsolutePathBuf },
}
```

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -209,13 +209,24 @@ v2_enum_from_core!(
 );
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
-#[serde(rename_all = "camelCase")]
+#[serde(tag = "type", rename_all = "camelCase")]
+#[ts(tag = "type")]
 #[ts(export_to = "v2/")]
-pub enum ConfigLayerName {
-    Mdm,
-    System,
+pub enum ConfigLayerSource {
+    /// Managed preferences layer delivered by MDM (macOS only).
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    Mdm { domain: String, key: String },
+    /// Managed config layer from a file (usually `managed_config.toml`).
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    System { file: AbsolutePathBuf },
+    /// Session-layer overrides supplied via `-c`/`--config`.
     SessionFlags,
-    User,
+    /// User config layer from a file (usually `config.toml`).
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    User { file: AbsolutePathBuf },
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema, TS)]
@@ -288,8 +299,7 @@ pub struct Config {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ConfigLayerMetadata {
-    pub name: ConfigLayerName,
-    pub source: String,
+    pub name: ConfigLayerSource,
     pub version: String,
 }
 
@@ -297,8 +307,7 @@ pub struct ConfigLayerMetadata {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ConfigLayer {
-    pub name: ConfigLayerName,
-    pub source: String,
+    pub name: ConfigLayerSource,
     pub version: String,
     pub config: JsonValue,
 }