`participantContextId` data migration

[ndr-brt]

WHAT

Since EDC version 0.15.0, there's this Participant Context concept, that will permit a single EDC runtime to manage multiple participants. In TractusX EDC tho, there will be the need only for one for the time being.
PR #2434 added a new participant_context_id column in a lot of different entity tables.

There's the need to have a configuration and data migration that will permit the already existing connectors to cope with the new model.

More info on:
https://github.com/eclipse-edc/Connector/tree/910fb7260e49a0408b4d31a5df3000ac20c4f506/docs/developer/decision-records/2025-08-26-participant-context#migration

[lgblaumeiser]

The current main has the corresponding columns in the tables in place, and there is a migration to add the column and fill it with NULL. We need a migration that adds a sensible random value.

An example migration
https://github.com/Mobility-Data-Space/mds-edc/blob/277e97aa386dae9166962da4d94e24f9d6f765d1/extensions/database-schema-migration-connector/src/main/resources/migrations/connector/V1_5_0__Set_ParticipantContextId.sql

An example extension to ensure proper setting
https://github.com/Mobility-Data-Space/mds-edc/blob/main/extensions/database-schema-migration-connector/src/main/java/eu/dataspace/connector/postgresql/migration/ConnectorPostgresqlMigration.java