Add test coverage for GOPACS UFTP V3 participant migration #40

Add GOPACSHandler tests covering the day-ahead capacity-steering message flow from the AGR (EMS) side, per the GOPACS "Flexibility trade (day-ahead) using own UFTP implementation" manual.

• FlexRequest received → asset updated from the request ISPs, and a FlexRequestResponse (Accepted) followed by a FlexOffer is sent back to the DSO.
• FlexOfferResponse received → accepted vs rejected handled (no further message sent).
• FlexOrder received → asset updated from the order ISPs, and a FlexOrderResponse is sent back.
• Out-of-scope message → a signed flex message whose CongestionPoint does not match the contracted EAN is dropped: no asset mutation and no reply. (Congestion point is the contract's grid-connection EAN; cross-operator activation means the sender may differ from the contract partner, so scoping keys on the EAN, not the sender.)