[UR][CUDA] Fix IL_VERSION query to include null terminator (#21600)
The UR_DEVICE_INFO_IL_VERSION query was returning PTX ISA version string
using ILVersion.data() and ILVersion.size(), which did not include the
null terminator. This caused test failures because the conformance test
expects the returned string to be null-terminated.
Changed to use ILVersion.c_str() to return a properly null-terminated
string, consistent with other string properties in the same file.
This fix enables the SuccessILVersion conformance test for CUDA adapter.