an expression for someone else taking over a project at the very end, then taking credit for it

Solution 1:

There's the expression steal someone's thunder:

[Merriam-Webster]
: to prevent someone from having success or getting attention, praise, etc., by doing or saying whatever that person was planning to do or say
// I didn't mean to steal your thunder, but I just had to tell your mom about your promotion.

While it's a broader term, it does also apply to the specific situation you describe.

Somebody could come into a project after all the hard work has been done, present the result to management, and reap the reward for having accomplished the task:

She came in after I'd done all the hard work and stole my thunder.