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/execpolicy/src/error.rs

@@ -3,6 +3,24 @@ use thiserror::Error;
 
 pub type Result<T> = std::result::Result<T, Error>;
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct TextPosition {
+    pub line: usize,
+    pub column: usize,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct TextRange {
+    pub start: TextPosition,
+    pub end: TextPosition,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ErrorLocation {
+    pub path: String,
+    pub range: TextRange,
+}
+
 #[derive(Debug, Error)]
 pub enum Error {
     #[error("invalid decision: {0}")]
@@ -26,3 +44,27 @@ pub enum Error {
     #[error("starlark error: {0}")]
     Starlark(StarlarkError),
 }
+
+impl Error {
+    pub fn location(&self) -> Option<ErrorLocation> {
+        match self {
+            Error::Starlark(err) => err.span().map(|span| {
+                let resolved = span.resolve_span();
+                ErrorLocation {
+                    path: span.filename().to_string(),
+                    range: TextRange {
+                        start: TextPosition {
+                            line: resolved.begin.line + 1,
+                            column: resolved.begin.column + 1,
+                        },
+                        end: TextPosition {
+                            line: resolved.end.line + 1,
+                            column: resolved.end.column + 1,
+                        },
+                    },
+                }
+            }),
+            _ => None,
+        }
+    }
+}

codex-rs/execpolicy/src/lib.rs

@@ -10,7 +10,10 @@ pub use amend::AmendError;
 pub use amend::blocking_append_allow_prefix_rule;
 pub use decision::Decision;
 pub use error::Error;
+pub use error::ErrorLocation;
 pub use error::Result;
+pub use error::TextPosition;
+pub use error::TextRange;
 pub use execpolicycheck::ExecPolicyCheckCommand;
 pub use parser::PolicyParser;
 pub use policy::Evaluation;