Throw an exception for duplicate interface port names

[aishasalim]

Fixes #514.

Interface._setPort caught a port name that already exists on the interface with an assert:

assert(!_ports.containsKey(portName),
    'Port named $portName already exists on this interface.');

Asserts are stripped in release mode, so outside of tests the duplicate silently replaced the earlier entry in _ports and the interface ended up carrying a port the caller never intended.

This replaces it with a DuplicatePortNameException, a new RohdException alongside the existing InterfaceNameException and InterfaceTypeException in lib/src/exceptions/interface/, so the collision is reported in every build mode.

Testing

New test in interface_test.dart builds an interface that declares the same port name twice and expects the exception. Confirmed it fails against unmodified interface.dart first — it threw _AssertionError rather than a RohdException — so the test genuinely covers the change.

Full gate locally: dart test 1320 passed, dart format 0 changed, dart analyze --fatal-infos clean, dart doc 0 warnings and 0 errors, pana 160/160.

Note

This is a behavior change for release-mode builds that were unknowingly relying on the silent overwrite — that is the point of the issue, but flagging it since it could surface in downstream code. Happy to rename the exception or reword the message if you would prefer something different.

[mkorbel1]

Looks great, thank you for the contribution!