fix: add db2 to FORBIDDEN_STATE_SYNC_ENGINES

Db2 rejects table names starting with underscore, which SQLMesh uses
for all state tables (_versions, _snapshots, _environments, _intervals).
Without this entry is_forbidden_for_state_sync() returns False for Db2,
allowing SQLMesh to silently attempt — and fail — to create state tables
instead of raising a clear ConfigError upfront.

The comment explaining the reason has been moved inside the set alongside
the entry it describes, consistent with the pattern used for the other
engines in the set.

Signed-off-by: IBM Db2 Eco System <Hdm-dev-persona-db2-eco-system@ibm.com>

Author: hdm-db2-eco-system

sqlmesh/core/config/connection.py

@@ -53,16 +53,16 @@
     "mssql",
     "azuresql",
 }
-# Note: Db2 is excluded because it doesn't allow table names starting with underscore (_)
-# which SQLMesh uses for state tables (_versions, _snapshots, _environments, _intervals).
-# Use a separate state_connection (e.g., DuckDB) for Db2 gateways.
 FORBIDDEN_STATE_SYNC_ENGINES = {
     # Do not support row-level operations
     "spark",
     "trino",
     # Nullable types are problematic
     "clickhouse",
     "starrocks",
+    # Db2 rejects table names starting with underscore (_versions, _snapshots,
+    # _environments, _intervals). Use a separate state_connection (e.g., DuckDB).
+    "db2",
 }
 MOTHERDUCK_TOKEN_REGEX = re.compile(r"(\?|\&)(motherduck_token=)(\S*)")
 PASSWORD_REGEX = re.compile(r"(password=)(\S+)")