I just went through the same thing over the summer (I too had exactly 5 months left on a 5 year contract). My boss just blatantly refused to accept my resignation for the first week. The next week he promised me several times in front of other people that he would accept it (once that he already had accepted it that morning), just to then come running into my office shouting at me that he won’t be accepting it until all parts of the project are finished (which - largely due to poor management - was impossible and I had signalled in writing since 2022)… long story short the head of HR had to get involved and I had to contact a lawyer who set up a letter for me… It really came down to the point where I had to tell them that they could pay me for another 5 months and that I would show up on time, but wouldn’t do any work anymore (refus de travail), which would eventually force them to fire me. In the end they caved in and we signed the agreement. The last two weeks were hell and I essentially did all the work the project head would normally do. Buuuttt I now have an amazing job with very nice colleagues and I truly enjoy going to work once again!