fix: connection interface end names dropped on textual SysML export

[mvanhorn]

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Interface-type connections now serialize their two ends when exporting to textual SysML, producing connection interface1 connect heater.socket to outlet; instead of connection interface1 connect to ;. #2264 (@AxelRICHARD acknowledged) reported that the textual export drops the end references for interface connections while Flow connections were unaffected, which pointed at a divergent branch in the connection-end serialization path. In SysMLElementSerializer, the interface-connection case did not emit the qualified end names the way the working flow branch does, so the connect <end1> to <end2> form came out empty; the fix resolves and emits each end's qualified feature name on that path. A serializer test covers an interface connection between two ports and asserts the exported text carries both qualified end names, with the existing flow-connection serialization unchanged.

Fixes #2264

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[AxelRICHARD]

Hello,

Thank you for providing this PR. Could you please add an entry to the CHANGELOG as required by the PR template?
Could you also signed the Eclipse Contributor Agreement (ECA) https://www.eclipse.org/legal/eca/ please ? I won't be able to merge the PR if you don't sign it.

Thank you again,

Regards,

[AxelRICHARD]

You also need to follow this rule https://doc.mbse-syson.org/syson/v2026.5.0/developer-guide/index.html#_contribute_a_change_in_the_codebase about the commit message.

Thank you