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/exec/tests/suite/originator.rs

@@ -1,6 +1,7 @@
 #![cfg(not(target_os = "windows"))]
 #![allow(clippy::expect_used, clippy::unwrap_used)]
 
+use codex_core::default_client::CODEX_INTERNAL_ORIGINATOR_OVERRIDE_ENV_VAR;
 use core_test_support::responses;
 use core_test_support::test_codex_exec::test_codex_exec;
 use wiremock::matchers::header;
@@ -20,6 +21,7 @@ async fn send_codex_exec_originator() -> anyhow::Result<()> {
     responses::mount_sse_once_match(&server, header("Originator", "codex_exec"), body).await;
 
     test.cmd_with_server(&server)
+        .env_remove(CODEX_INTERNAL_ORIGINATOR_OVERRIDE_ENV_VAR)
         .arg("--skip-git-repo-check")
         .arg("tell me something")
         .assert()