Bus merging logic to ensure correct bus type updates

[jbarberia]

Proposed fix for #935 and #936.

This update addresses an issue with the bus merging logic to ensure the resulting bus_type is correctly updated when buses are merged.

When two buses are connected by a switch or a zero-impedance line, the merged bus should have the highest priority bus_type (PQ=1, PV=2, Slack=3)

Changes Introduced

• Correctly updates the bus_type of the resulting bus to reflect the highest priority among the merged buses.
• Adds safeguards to prevent assigning invalid bus types (e.g., type 4).