fix(tui): highlight C++ module files (#28554)

## Why

Codex syntax-highlights diffs for conventional C++ extensions such as
`.cpp` and `.cxx`, but C++ module interface files using `.cppm`, `.ixx`,
or `.cxxm` fall back to plain diff coloring. The bundled syntax set
already includes C++, but it does not resolve those module extensions by
itself.

Closes #28223.

## What changed

- map `.cppm`, `.ixx`, and `.cxxm` to the existing `cpp` syntax in
`render/highlight.rs`
- extend alias-resolution coverage for all three module extensions
- verify `.cpp`, `.cppm`, `.ixx`, and `.cxxm` diffs produce
syntax-highlighted RGB spans while unknown extensions retain the plain
fallback
- snapshot the syntax-colored token segmentation for the supported C++
module extensions

## How to Test

1. Ask Codex to create or modify a C++ module interface file using
`.cppm`, `.ixx`, or `.cxxm`.
2. Confirm C++ tokens in the rendered diff receive syntax colors instead
of only the red/green diff treatment.
3. Modify an equivalent `.cpp` file and confirm its existing
highlighting remains unchanged.
4. Modify a file with an unknown extension and confirm it still uses the
plain diff fallback.

Targeted tests:

- `just test -p codex-tui -E
'test(find_syntax_resolves_languages_and_aliases) |
test(cpp_module_extensions_use_cpp_highlighting) |
test(unknown_extension_falls_back_without_syntax_highlighting)'`

codex-rs/tui/src/diff_render.rs

@@ -1307,6 +1307,7 @@ fn style_gutter_dim() -> Style {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use insta::assert_debug_snapshot;
     use insta::assert_snapshot;
     use pretty_assertions::assert_eq;
     use ratatui::Terminal;
@@ -2200,6 +2201,58 @@ mod tests {
         );
     }
 
+    #[test]
+    fn cpp_module_extensions_use_cpp_highlighting() {
+        let highlighted_tokens = ["cpp", "cppm", "CPPM", "cxxm", "CxXm", "ixx", "IXX"]
+            .into_iter()
+            .map(|extension| {
+                let mut changes: HashMap<PathBuf, FileChange> = HashMap::new();
+                changes.insert(
+                    PathBuf::from(format!("math.{extension}")),
+                    FileChange::Add {
+                        content:
+                            "export module math;\nexport int sum(int a, int b) { return a + b; }\n"
+                                .to_string(),
+                    },
+                );
+
+                let lines =
+                    create_diff_summary(&changes, &PathBuf::from("/"), /*wrap_cols*/ 80);
+                let rgb_tokens = lines
+                    .iter()
+                    .flat_map(|line| &line.spans)
+                    .filter(|span| matches!(span.style.fg, Some(ratatui::style::Color::Rgb(..))))
+                    .map(|span| span.content.to_string())
+                    .collect::<Vec<_>>();
+                assert!(
+                    !rgb_tokens.is_empty(),
+                    "add diff for .{extension} file should produce syntax-highlighted (RGB) spans"
+                );
+                (extension, rgb_tokens.join("|"))
+            })
+            .collect::<Vec<_>>();
+
+        assert_debug_snapshot!("cpp_module_extension_highlighting", highlighted_tokens);
+    }
+
+    #[test]
+    fn unknown_extension_falls_back_without_syntax_highlighting() {
+        let mut changes: HashMap<PathBuf, FileChange> = HashMap::new();
+        changes.insert(
+            PathBuf::from("math.unknown-extension"),
+            FileChange::Add {
+                content: "export module math;\nexport int value = 42;\n".to_string(),
+            },
+        );
+
+        let lines = create_diff_summary(&changes, &PathBuf::from("/"), /*wrap_cols*/ 80);
+        assert!(lines.iter().all(|line| {
+            line.spans
+                .iter()
+                .all(|span| !matches!(span.style.fg, Some(ratatui::style::Color::Rgb(..))))
+        }));
+    }
+
     #[test]
     fn delete_diff_uses_path_extension_for_highlighting() {
         let mut changes: HashMap<PathBuf, FileChange> = HashMap::new();