Fix warnings and disable stack_memory_file test for now @twogood

[twogood]

No description provided.

[Copilot]

Pull request overview

This PR aims to address compiler warnings and temporarily disable a problematic test while tightening CI compilation flags.

Changes:

• Comments out the v5/stack_memory_file test subdirectory from the CTest configuration.
• Modifies unshield_utf16_to_utf8 in lib/helper.c to remove an unused variable when calling utf16_to_utf8.
• Updates the GitHub Actions CMake workflow to attempt to enable -Wall -Werror during CI builds.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
test/CMakeLists.txt	Disables the v5/stack_memory_file test suite by commenting out its subdirectory inclusion.
lib/helper.c	Adjusts UTF-16 to UTF-8 conversion helper to allocate directly into string_buffer->string and remove an unused result variable, but introduces a malformed utf16_to_utf8 call.
.github/workflows/cmake.yml	Attempts to enforce -Wall -Werror via a new CFLAGS cache variable in the CMake configure step, though it is not currently wired into the CMake build.

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[twogood]

Haha, embarrassing, tried to get help from Copilot...