Fix replace partition error from version >26.6

[koletzilla]

Summary

Fix the insert_overwrite incremental strategy failing on ClickHouse 26.6+ with Code: 36 ... refusing REPLACE PARTITION because it would silently drop the destination partition's data. The strategy intentionally replaces partitions from a source table that may have no parts for a partition (to clear shards that received no new data on distributed tables), which newer servers reject by default. The REPLACE PARTITION statement now includes allow_replace_partition_from_empty_source=1 on servers 26.6 and newer; older servers keep their existing behavior.

I went with the simplest solution: Just add the setting for this specific use case as we explicitly want this one to behave this way. Not touching other queries in case we want to have a different behaviour for them.

Avoided other possible implementations with the focus to keep it simple:

• Checking if the setting is available by checking existing settings: Just checking by version is enough.
• Adding it to the list of current settings applied: Many required changes to do that for just this one.

[Copilot]

Pull request overview

This PR fixes the insert_overwrite incremental strategy for ClickHouse 26.6+ by conditionally enabling a new server setting required to allow REPLACE PARTITION operations when the source partition is empty on some shards (common in distributed scenarios).

Changes:

• Add SETTINGS allow_replace_partition_from_empty_source = 1 to the REPLACE PARTITION ALTER TABLE statement when running on ClickHouse >= 26.6.
• Document the fix in the changelog under “Unreleased / Bug Fixes”.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dbt/include/clickhouse/macros/materializations/incremental/incremental.sql	Adds a ClickHouse-version-gated SETTINGS allow_replace_partition_from_empty_source = 1 clause to the REPLACE PARTITION statement used by insert_overwrite.
CHANGELOG.md	Adds an Unreleased changelog entry describing the ClickHouse 26.6+ fix for insert_overwrite.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[koletzilla]

test_cloud error is another flaky test. Solving it here: #681