Resolve "Bug in exam lab request handling: requests are rejected too soon"

What does this mr do?

Closes #349 (closed) Fixes bug where all requests in an exam lab would get rejected.

Actions taken to fix bug

Now, whenever the first request is picked using 'get next', the requests in that timeslot that did not get picked will be rejected.

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.