With a server, you exchange money for goods & services. Tipping makes the exchange a little weird and tax laws built around the practice can be complex. But it's still fundamentally a "normal" purchase. Not corruption.

With a government agent, you are not necessarily supposed to be paying them. Typical bribes are dependent upon an implicit threat of abuse of power rather than being a normal purchase of goods & services.

Tips are not a "lesser form of corruption". Unless you were somehow buying something with that tip that would otherwise be off-limits but in that case I think people would call it a bribe.