Codegen insert-trigger validations ignore column nullable flag

In the unified domain-entity org model, the * Insert trigger / * Validations table wires a column straight to a validation function (e.g. country_alpha2_code -> ores_refdata_validate_country_fn). The template (sql_schema_domain_entity_create.mustache, around the insert_trigger.validations loop) already has a nullable branch that skips the call when the column is null, but org_loader.py's _parse_org_table_rows for the Validations section never cross-references each row's column against that field's own :nullable: property from the model — so every row is rendered as if non-nullable, and the guard never fires. Fix: when building de["insert_trigger"]["validations"] rows in projects/ores.codegen/src/codegen/org_loader.py, look up each row's column in the entity's field list and copy its nullable flag onto the row so the template's existing nullable branch actually engages.

Found while regenerating business_centre (doc/agile/versions/v0/sprint_23/commission_business_centre/task_regenerate-business-centre-reconcile-diffs.org): country_alpha2_code is :nullable: true in the model but its generated insert trigger (projects/ores.sql/create/refdata/refdata_business_centres_create.sql) called ores_refdata_validate_country_fn unconditionally, which raises on null/empty input — breaking the system 'WRLD' business centre (no country) and any future nullable+validated field on any entity. Worked around by hand-patching the generated SQL for this regeneration; the generator itself still has the bug and will reintroduce it on the next regenerate.

• projects/ores.codegen/src/codegen/org_loader.py (Insert trigger / Validations parsing, ~line 750)
• projects/ores.codegen/library/templates/sql_schema_domain_entity_create.mustache (~line 505, existing nullable branch that never gets engaged)
• projects/ores.sql/create/refdata/refdata_business_centres_create.sql (hand-patched instance of the bug)
• projects/ores.sql/create/refdata/refdata_counterparty_contact_informations_create.sql (existing manual workaround pattern for the same problem on country_code)

• Commission: business_centre
• Regenerate business_centre from its codegen model and reconcile diffs