Whether it was an internship or thesis shouldn't exactly matter for the CV.
What matters to the employers is what interesting skills have you developed during that time which they are going to want out of you.
List it as a student intern, but then go on about what you personally gained from it and can take with you to your employer:
Thingy Developer/Researcher - Student Intern from d/m/y to d/m6/y
applied thingamajigs pertaining the
x engineering project for 6 months
- Worked/Communicated with a team to deliver results on time
w,y,z concepts which brought
- Reported directly to project coordinator through
- Tasked with writing and documenting procedures for writing own bachelor's thesis and report.
Where you want to work also matters when writing this, as some skills may be more attractive than others depending on the position.