fix: replace instead of drop on managed Iceberg full refresh

[moomindani]

Resolves #1662

Description

A full refresh of an incremental model on a Unity Catalog managed Iceberg table ran drop table if exists and then the CTAS, so the table was absent for the whole rebuild — minutes on a large table — and concurrent queries failed with TABLE_OR_VIEW_NOT_FOUND.

The replaceability check keyed its Iceberg arm off file_format:

{% set is_iceberg = (catalog_relation.file_format == 'iceberg' and existing_relation.is_iceberg) %}

An Iceberg model never has file_format == 'iceberg'. iceberg_table_properties raises for any file_format other than delta, and iceberg is not in the accepted file_format list at all, so this arm was unreachable. The Delta arm then failed too, because the existing table reports Provider = iceberg. Both false, so the macro dropped a table that CREATE OR REPLACE handles fine — which is exactly the statement it ran next.

This keys the arm off table_format plus the use_managed_iceberg flag, the same condition file_format_clause uses to decide on using iceberg. The flag is read through get_behavior_flag_no_warn so projects that never opt in do not get a warning (#1266).

The Delta arm is unchanged, so a project that has just switched the flag on and still has a Delta table keeps replacing rather than dropping. The only case whose behaviour changes is managed Iceberg over managed Iceberg.

The predicate moved into format_allows_create_or_replace so the V1 and V2 paths share one definition and it can be tested without a warehouse.

Testing

The functional test asserts on the table's history rather than its id or creation time — both of those change on create or replace too, so only the first history entry surviving distinguishes a replace from a drop and recreate. It fails on the unfixed macro with history restarted, so the full refresh dropped and recreated the table and passes with the fix.

Against a serverless SQL warehouse: tests/functional/adapter/iceberg/ 11 passed, tests/functional/adapter/incremental/ 108 passed with 3 failures that reproduce identically on main (TestAppendParquet, and the two column-tag tests). Unit suite 1349 passed; code-quality clean.

Checklist

• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section.
• [Optional] I have run the dbt-databricks-pr-ready project skill for this PR and addressed its merge-readiness feedback