Fix serialization of invalid chars in C

[chvp]

Also took the opportunity to format the serialization a bit better.

Invalid characters are now treated as unsigned chars and written out in hex format (e.g. 0xA4). Better suggestions are welcome.

[bmesuere]

I think the C++ implementation needs a similar fix?

[Copilot]

Pull request overview

This pull request addresses serialization issues with invalid characters in C by implementing hex formatting for characters with the high bit set. The PR also improves code formatting in the escape function and adds a test for detecting wrong function signatures in C.

Changes:

• Modified the escape() function to output high-bit characters in hex format (e.g., 0xA4) instead of attempting standard escaping
• Improved code formatting and structure in the escape function
• Added test case for detecting C function signature mismatches (returning wrong type)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tested/languages/c/templates/values.c	Updated escape function to handle high-bit characters with hex formatting; improved formatting and memory allocation
tests/test_language_quirks.py	Added test for detecting wrong function return type in C
tests/exercises/echo-function/solution/wrong-signature.c	Added test solution file with intentional type mismatch (returns char instead of char*)

Comments suppressed due to low confidence (2)

tests/test_language_quirks.py:29

• The test name test_c_pointer_char_return doesn't clearly describe what's being tested. Consider renaming to something more descriptive like test_c_wrong_return_type_signature or test_c_return_type_mismatch_detection to better convey that this test verifies the system correctly detects when a function returns the wrong type (char instead of char*).
  tests/test_language_quirks.py:40
• The new test test_c_pointer_char_return is testing for a wrong function signature (returning char instead of char*), but it doesn't actually test the new hex formatting functionality for invalid characters that was added in this PR. Consider adding a test case that passes a string containing high-bit or non-printable characters to verify that the hex formatting (e.g., 0xA4) works correctly.

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[chvp]

At least not the same fix; the serialization uses std::string, not sure how that behaves. https://github.com/dodona-edu/universal-judge/blob/master/tested/languages/cpp/templates/values.tpp#L66

[chvp]

Did some tests, this is much harder to trigger in cpp, but still possible, so I added a fix for that serializer as well.

[niknetniko]

Seems a reasonable solution to me.