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/v2/config_rpc.rs

@@ -6,7 +6,7 @@ use app_test_support::to_response;
 use codex_app_server_protocol::AskForApproval;
 use codex_app_server_protocol::ConfigBatchWriteParams;
 use codex_app_server_protocol::ConfigEdit;
-use codex_app_server_protocol::ConfigLayerName;
+use codex_app_server_protocol::ConfigLayerSource;
 use codex_app_server_protocol::ConfigReadParams;
 use codex_app_server_protocol::ConfigReadResponse;
 use codex_app_server_protocol::ConfigValueWriteParams;
@@ -18,6 +18,7 @@ use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::SandboxMode;
 use codex_app_server_protocol::ToolsV2;
 use codex_app_server_protocol::WriteStatus;
+use codex_utils_absolute_path::AbsolutePathBuf;
 use pretty_assertions::assert_eq;
 use serde_json::json;
 use tempfile::TempDir;
@@ -42,6 +43,8 @@ model = "gpt-user"
 sandbox_mode = "workspace-write"
 "#,
     )?;
+    let codex_home_path = codex_home.path().canonicalize()?;
+    let user_file = AbsolutePathBuf::try_from(codex_home_path.join("config.toml"))?;
 
     let mut mcp = McpProcess::new(codex_home.path()).await?;
     timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
@@ -65,12 +68,14 @@ sandbox_mode = "workspace-write"
     assert_eq!(config.model.as_deref(), Some("gpt-user"));
     assert_eq!(
         origins.get("model").expect("origin").name,
-        ConfigLayerName::User
+        ConfigLayerSource::User {
+            file: user_file.clone(),
+        }
     );
     let layers = layers.expect("layers present");
     assert_eq!(layers.len(), 2);
-    assert_eq!(layers[0].name, ConfigLayerName::SessionFlags);
-    assert_eq!(layers[1].name, ConfigLayerName::User);
+    assert_eq!(layers[0].name, ConfigLayerSource::SessionFlags);
+    assert_eq!(layers[1].name, ConfigLayerSource::User { file: user_file });
 
     Ok(())
 }
@@ -88,6 +93,8 @@ web_search = true
 view_image = false
 "#,
     )?;
+    let codex_home_path = codex_home.path().canonicalize()?;
+    let user_file = AbsolutePathBuf::try_from(codex_home_path.join("config.toml"))?;
 
     let mut mcp = McpProcess::new(codex_home.path()).await?;
     timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
@@ -118,17 +125,21 @@ view_image = false
     );
     assert_eq!(
         origins.get("tools.web_search").expect("origin").name,
-        ConfigLayerName::User
+        ConfigLayerSource::User {
+            file: user_file.clone(),
+        }
     );
     assert_eq!(
         origins.get("tools.view_image").expect("origin").name,
-        ConfigLayerName::User
+        ConfigLayerSource::User {
+            file: user_file.clone(),
+        }
     );
 
     let layers = layers.expect("layers present");
     assert_eq!(layers.len(), 2);
-    assert_eq!(layers[0].name, ConfigLayerName::SessionFlags);
-    assert_eq!(layers[1].name, ConfigLayerName::User);
+    assert_eq!(layers[0].name, ConfigLayerSource::SessionFlags);
+    assert_eq!(layers[1].name, ConfigLayerSource::User { file: user_file });
 
     Ok(())
 }
@@ -153,8 +164,11 @@ network_access = true
             serde_json::json!(user_dir)
         ),
     )?;
+    let codex_home_path = codex_home.path().canonicalize()?;
+    let user_file = AbsolutePathBuf::try_from(codex_home_path.join("config.toml"))?;
 
     let managed_path = codex_home.path().join("managed_config.toml");
+    let managed_file = AbsolutePathBuf::try_from(managed_path.clone())?;
     std::fs::write(
         &managed_path,
         format!(
@@ -197,19 +211,25 @@ writable_roots = [{}]
     assert_eq!(config.model.as_deref(), Some("gpt-system"));
     assert_eq!(
         origins.get("model").expect("origin").name,
-        ConfigLayerName::System
+        ConfigLayerSource::System {
+            file: managed_file.clone(),
+        }
     );
 
     assert_eq!(config.approval_policy, Some(AskForApproval::Never));
     assert_eq!(
         origins.get("approval_policy").expect("origin").name,
-        ConfigLayerName::System
+        ConfigLayerSource::System {
+            file: managed_file.clone(),
+        }
     );
 
     assert_eq!(config.sandbox_mode, Some(SandboxMode::WorkspaceWrite));
     assert_eq!(
         origins.get("sandbox_mode").expect("origin").name,
-        ConfigLayerName::User
+        ConfigLayerSource::User {
+            file: user_file.clone(),
+        }
     );
 
     let sandbox = config
@@ -222,7 +242,9 @@ writable_roots = [{}]
             .get("sandbox_workspace_write.writable_roots.0")
             .expect("origin")
             .name,
-        ConfigLayerName::System
+        ConfigLayerSource::System {
+            file: managed_file.clone(),
+        }
     );
 
     assert!(sandbox.network_access);
@@ -231,14 +253,19 @@ writable_roots = [{}]
             .get("sandbox_workspace_write.network_access")
             .expect("origin")
             .name,
-        ConfigLayerName::User
+        ConfigLayerSource::User {
+            file: user_file.clone(),
+        }
     );
 
     let layers = layers.expect("layers present");
     assert_eq!(layers.len(), 3);
-    assert_eq!(layers[0].name, ConfigLayerName::System);
-    assert_eq!(layers[1].name, ConfigLayerName::SessionFlags);
-    assert_eq!(layers[2].name, ConfigLayerName::User);
+    assert_eq!(
+        layers[0].name,
+        ConfigLayerSource::System { file: managed_file }
+    );
+    assert_eq!(layers[1].name, ConfigLayerSource::SessionFlags);
+    assert_eq!(layers[2].name, ConfigLayerSource::User { file: user_file });
 
     Ok(())
 }