fix(tui): highlight C++ module files

[fcoury-oai]

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)'

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 11b37e75d6

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[fcoury-oai]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 13b40b2b4b

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[fcoury-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 8488219425

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[etraut-openai]

Code looks good.

I didn't bother building or manually testing this one since it's pretty simple and has low regression risk.