For reference, a pending build looks like this:

There's no stop, start, restart, cancel, delete buttons. It's difficult to recreate this scenario, but you can do it by spinning up an tagged external provisioner, publishing a template that uses that provisioner tag, then stopping the provisioner and creating a workspace.

Ideally the frontend can send a cancel request with a precondition on the request that the server ensures the build isn't started or claimed when being cancelled. We do not want a race between the user clicking cancel on a pending build and the build actually being picked up.

The server should accept a query parameter or header that denotes the job cancellation should only be processed if it's pending, and the server should take care to lock the rows and use a transaction to prevent it from being picked up during the request.

related to #17818