ci: suppress GCC 15 basic_string UBSan path

[xsscx]

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[Copilot]

Pull request overview

Updates the runtime UBSan suppression list to account for GCC 15/libstdc++ emitting a non-canonical include path (with ../../.. segments) for basic_string.tcc, preventing CI noise from standard-library frames.

Changes:

• Add a comment documenting the GCC 15 non-canonical path behavior.
• Add an additional unsigned-integer-overflow suppression entry matching the non-canonical basic_string.tcc path.