Unset protobuf input port zero can fail worker port lookup

### Feature Summary

Input port zero can arrive from protobuf as an unset `PortIdentity()`. It compares equal to the registered `PortIdentity(id=0, internal=False)`, but the generated messages have different hashes. `InputManager.get_port` uses direct dictionary lookup, so the equal incoming identity raises `KeyError`.

Before: an unset protobuf identity for input port zero can fail lookup.

Expected: input ports resolve by protobuf value equality, while unknown ports still fail clearly.

Reproduction evidence:

Version and commit evidence:

### Proposed Solution or Design

Expected: input ports resolve by protobuf value equality, while unknown ports still fail clearly.

### Affected Area

Workflow Engine (Amber)