[HL2MP] Update func_tank transmission rules

[speedvoltage]

Issue: The func_tank entity was not being properly transmitted to clients under certain conditions, causing issues where it would not render or function correctly in multiplayer. This was likely due to improper ShouldTransmit settings, preventing the entity from being networked in all necessary cases.

Fix: Implemented a proper ShouldTransmit function for func_tank to ensure it is always transmitted (FL_EDICT_ALWAYS). Additionally, added UpdateTransmitState() to make sure it follows the expected network transmission behavior, resolving visibility and interaction issues.