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/tests/suite/user_agent.rs

@@ -25,12 +25,13 @@ async fn get_user_agent_returns_current_codex_user_agent() -> Result<()> {
     .await??;
 
     let os_info = os_info::get();
+    let originator = codex_core::default_client::originator().value.as_str();
+    let os_type = os_info.os_type();
+    let os_version = os_info.version();
+    let architecture = os_info.architecture().unwrap_or("unknown");
+    let terminal_ua = codex_core::terminal::user_agent();
     let user_agent = format!(
-        "codex_cli_rs/0.0.0 ({} {}; {}) {} (codex-app-server-tests; 0.1.0)",
-        os_info.os_type(),
-        os_info.version(),
-        os_info.architecture().unwrap_or("unknown"),
-        codex_core::terminal::user_agent()
+        "{originator}/0.0.0 ({os_type} {os_version}; {architecture}) {terminal_ua} (codex-app-server-tests; 0.1.0)"
     );
 
     let received: GetUserAgentResponse = to_response(response)?;