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/mod.rs

@@ -410,9 +410,21 @@ impl ConfigBuilder {
         // relative paths to absolute paths based on the parent folder of the
         // respective config file, so we should be safe to deserialize without
         // AbsolutePathBufGuard here.
-        let config_toml: ConfigToml = merged_toml
-            .try_into()
-            .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))?;
+        let config_toml: ConfigToml = match merged_toml.try_into() {
+            Ok(config_toml) => config_toml,
+            Err(err) => {
+                if let Some(config_error) =
+                    crate::config_loader::first_layer_config_error(&config_layer_stack).await
+                {
+                    return Err(crate::config_loader::io_error_from_config_error(
+                        std::io::ErrorKind::InvalidData,
+                        config_error,
+                        Some(err),
+                    ));
+                }
+                return Err(std::io::Error::new(std::io::ErrorKind::InvalidData, err));
+            }
+        };
         Config::load_config_with_layer_stack(
             config_toml,
             harness_overrides,