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/mod.rs

@@ -1,4 +1,5 @@
 mod config_requirements;
+mod diagnostics;
 mod fingerprint;
 mod layer_io;
 #[cfg(target_os = "macos")]
@@ -34,6 +35,16 @@ pub use config_requirements::McpServerRequirement;
 pub use config_requirements::RequirementSource;
 pub use config_requirements::SandboxModeRequirement;
 pub use config_requirements::Sourced;
+pub use diagnostics::ConfigError;
+pub use diagnostics::ConfigLoadError;
+pub use diagnostics::TextPosition;
+pub use diagnostics::TextRange;
+pub(crate) use diagnostics::config_error_from_toml;
+pub(crate) use diagnostics::first_layer_config_error;
+pub(crate) use diagnostics::first_layer_config_error_from_entries;
+pub use diagnostics::format_config_error;
+pub use diagnostics::format_config_error_with_source;
+pub(crate) use diagnostics::io_error_from_config_error;
 pub use merge::merge_toml_values;
 pub(crate) use overrides::build_cli_overrides_layer;
 pub use state::ConfigLayerEntry;
@@ -171,16 +182,44 @@ pub async fn load_config_layers_state(
             merge_toml_values(&mut merged_so_far, cli_overrides_layer);
         }
 
-        let project_root_markers = project_root_markers_from_config(&merged_so_far)?
-            .unwrap_or_else(default_project_root_markers);
-        let project_trust_context = project_trust_context(
+        let project_root_markers = match project_root_markers_from_config(&merged_so_far) {
+            Ok(markers) => markers.unwrap_or_else(default_project_root_markers),
+            Err(err) => {
+                if let Some(config_error) = first_layer_config_error_from_entries(&layers).await {
+                    return Err(io_error_from_config_error(
+                        io::ErrorKind::InvalidData,
+                        config_error,
+                        None,
+                    ));
+                }
+                return Err(err);
+            }
+        };
+        let project_trust_context = match project_trust_context(
             &merged_so_far,
             &cwd,
             &project_root_markers,
             codex_home,
             &user_file,
         )
-        .await?;
+        .await
+        {
+            Ok(context) => context,
+            Err(err) => {
+                let source = err
+                    .get_ref()
+                    .and_then(|err| err.downcast_ref::<toml::de::Error>())
+                    .cloned();
+                if let Some(config_error) = first_layer_config_error_from_entries(&layers).await {
+                    return Err(io_error_from_config_error(
+                        io::ErrorKind::InvalidData,
+                        config_error,
+                        source,
+                    ));
+                }
+                return Err(err);
+            }
+        };
         let project_layers = load_project_layers(
             &cwd,
             &project_trust_context.project_root,
@@ -252,11 +291,9 @@ async fn load_config_toml_for_required_layer(
     let toml_file = config_toml.as_ref();
     let toml_value = match tokio::fs::read_to_string(toml_file).await {
         Ok(contents) => {
-            let config: TomlValue = toml::from_str(&contents).map_err(|e| {
-                io::Error::new(
-                    io::ErrorKind::InvalidData,
-                    format!("Error parsing config file {}: {e}", toml_file.display()),
-                )
+            let config: TomlValue = toml::from_str(&contents).map_err(|err| {
+                let config_error = config_error_from_toml(toml_file, &contents, err.clone());
+                io_error_from_config_error(io::ErrorKind::InvalidData, config_error, Some(err))
             })?;
             let config_parent = toml_file.parent().ok_or_else(|| {
                 io::Error::new(