Another round of improvements for config error messages (#9746)

In a [recent PR](https://github.com/openai/codex/pull/9182), I made some
improvements to config error messages so errors didn't leave app server
clients in a dead state. This is a follow-on PR to make these error
messages more readable and actionable for both TUI and GUI users. For
example, see #9668 where the user was understandably confused about the
source of the problem and how to fix it.

The improved error message:
1. Clearly identifies the config file where the error was found (which
is more important now that we support layered configs)
2. Provides a line and column number of the error
3. Displays the line where the error occurred and underlines it

For example, if my `config.toml` includes the following:
```toml
[features]
collaboration_modes = "true"
```

Here's the current CLI error message:
```
Error loading config.toml: invalid type: string "true", expected a boolean in `features`
```

And here's the improved message:
```
Error loading config.toml:
/Users/etraut/.codex/config.toml:43:23: invalid type: string "true", expected a boolean
   |
43 | collaboration_modes = "true"
   |                       ^^^^^^
```

The bulk of the new logic is contained within a new module
`config_loader/diagnostics.rs` that is responsible for calculating the
text range for a given toml path (which is more involved than I would
have expected).

In addition, this PR adds the file name and text range to the
`ConfigWarningNotification` app server struct. This allows GUI clients
to present the user with a better error message and an optional link to
open the errant config file. This was a suggestion from @.bolinfest when
he reviewed my previous PR.

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

@@ -6,6 +6,7 @@ use crate::config::ConfigOverrides;
 use crate::config::ConfigToml;
 use crate::config::ProjectConfig;
 use crate::config_loader::ConfigLayerEntry;
+use crate::config_loader::ConfigLoadError;
 use crate::config_loader::ConfigRequirements;
 use crate::config_loader::config_requirements::ConfigRequirementsWithSources;
 use crate::config_loader::fingerprint::version_for_toml;
@@ -21,6 +22,13 @@ use std::path::Path;
 use tempfile::tempdir;
 use toml::Value as TomlValue;
 
+fn config_error_from_io(err: &std::io::Error) -> &super::ConfigError {
+    err.get_ref()
+        .and_then(|err| err.downcast_ref::<ConfigLoadError>())
+        .map(ConfigLoadError::config_error)
+        .expect("expected ConfigLoadError")
+}
+
 async fn make_config_for_test(
     codex_home: &Path,
     project_path: &Path,
@@ -44,6 +52,98 @@ async fn make_config_for_test(
     .await
 }
 
+#[tokio::test]
+async fn returns_config_error_for_invalid_user_config_toml() {
+    let tmp = tempdir().expect("tempdir");
+    let contents = "model = \"gpt-4\"\ninvalid = [";
+    let config_path = tmp.path().join(CONFIG_TOML_FILE);
+    std::fs::write(&config_path, contents).expect("write config");
+
+    let cwd = AbsolutePathBuf::try_from(tmp.path()).expect("cwd");
+    let err = load_config_layers_state(
+        tmp.path(),
+        Some(cwd),
+        &[] as &[(String, TomlValue)],
+        LoaderOverrides::default(),
+    )
+    .await
+    .expect_err("expected error");
+
+    let config_error = config_error_from_io(&err);
+    let expected_toml_error = toml::from_str::<TomlValue>(contents).expect_err("parse error");
+    let expected_config_error =
+        super::config_error_from_toml(&config_path, contents, expected_toml_error);
+    assert_eq!(config_error, &expected_config_error);
+}
+
+#[tokio::test]
+async fn returns_config_error_for_invalid_managed_config_toml() {
+    let tmp = tempdir().expect("tempdir");
+    let managed_path = tmp.path().join("managed_config.toml");
+    let contents = "model = \"gpt-4\"\ninvalid = [";
+    std::fs::write(&managed_path, contents).expect("write managed config");
+
+    let overrides = LoaderOverrides {
+        managed_config_path: Some(managed_path.clone()),
+        ..Default::default()
+    };
+
+    let cwd = AbsolutePathBuf::try_from(tmp.path()).expect("cwd");
+    let err = load_config_layers_state(
+        tmp.path(),
+        Some(cwd),
+        &[] as &[(String, TomlValue)],
+        overrides,
+    )
+    .await
+    .expect_err("expected error");
+
+    let config_error = config_error_from_io(&err);
+    let expected_toml_error = toml::from_str::<TomlValue>(contents).expect_err("parse error");
+    let expected_config_error =
+        super::config_error_from_toml(&managed_path, contents, expected_toml_error);
+    assert_eq!(config_error, &expected_config_error);
+}
+
+#[tokio::test]
+async fn returns_config_error_for_schema_error_in_user_config() {
+    let tmp = tempdir().expect("tempdir");
+    let contents = "model_context_window = \"not_a_number\"";
+    let config_path = tmp.path().join(CONFIG_TOML_FILE);
+    std::fs::write(&config_path, contents).expect("write config");
+
+    let err = ConfigBuilder::default()
+        .codex_home(tmp.path().to_path_buf())
+        .fallback_cwd(Some(tmp.path().to_path_buf()))
+        .build()
+        .await
+        .expect_err("expected error");
+
+    let config_error = config_error_from_io(&err);
+    let _guard = codex_utils_absolute_path::AbsolutePathBufGuard::new(tmp.path());
+    let expected_config_error =
+        super::diagnostics::config_error_from_config_toml(&config_path, contents)
+            .expect("schema error");
+    assert_eq!(config_error, &expected_config_error);
+}
+
+#[test]
+fn schema_error_points_to_feature_value() {
+    let tmp = tempdir().expect("tempdir");
+    let contents = "[features]\ncollaboration_modes = \"true\"";
+    let config_path = tmp.path().join(CONFIG_TOML_FILE);
+    std::fs::write(&config_path, contents).expect("write config");
+
+    let _guard = codex_utils_absolute_path::AbsolutePathBufGuard::new(tmp.path());
+    let error = super::diagnostics::config_error_from_config_toml(&config_path, contents)
+        .expect("schema error");
+
+    let value_line = contents.lines().nth(1).expect("value line");
+    let value_column = value_line.find("\"true\"").expect("value") + 1;
+    assert_eq!(error.range.start.line, 2);
+    assert_eq!(error.range.start.column, value_column);
+}
+
 #[tokio::test]
 async fn merges_managed_config_layer_on_top() {
     let tmp = tempdir().expect("tempdir");