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/Cargo.toml

@@ -64,6 +64,7 @@ reqwest = { workspace = true, features = ["json", "stream"] }
 schemars = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
+serde_path_to_error = { workspace = true }
 serde_yaml = { workspace = true }
 sha1 = { workspace = true }
 sha2 = { workspace = true }