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/app-server-protocol/src/protocol/v2.rs

@@ -2503,6 +2503,24 @@ pub struct DeprecationNoticeNotification {
     pub details: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct TextPosition {
+    /// 1-based line number.
+    pub line: usize,
+    /// 1-based column number (in Unicode scalar values).
+    pub column: usize,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct TextRange {
+    pub start: TextPosition,
+    pub end: TextPosition,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
@@ -2511,6 +2529,14 @@ pub struct ConfigWarningNotification {
     pub summary: String,
     /// Optional extra guidance or error details.
     pub details: Option<String>,
+    /// Optional path to the config file that triggered the warning.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub path: Option<String>,
+    /// Optional range for the error location inside the config file.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub range: Option<TextRange>,
 }
 
 #[cfg(test)]