test(indexer): cover C++ module extensions

[cnYui]

Summary

• Add C++ language regression tests for C++20 module file extensions.
• Cover .cppm, .ixx, .mxx, .ccm, and .cxxm through Cpp::get_file_extensions, FileUtils::detect_language, and resolution_utils::detect_language_from_path.

Context

This adds test coverage for #50. Current master already recognizes these extensions; this PR locks the behavior down so module files keep being indexed as C++.

Verification

• rustup run 1.95-x86_64-pc-windows-msvc cargo fmt --check
• git diff --check

Attempted:

• $env:PROTOC = .tmp-tools/protoc/bin/protoc.exe; rustup run 1.95-x86_64-pc-windows-msvc cargo test cpp_module_extensions

The test build progressed with rustc 1.95 and a local protoc, but native Windows linking failed because this machine's SDK/link environment is missing DXCORE.lib required by DirectML/ORT dependencies. The failure occurs at linker setup, not in the new C++ extension test assertions.

Refs #50