feat: Add informix to DatabaseType enum

Informix is a standard relational SQL database (host/port/database/
user/password/schema), so it routes through the default
DatabaseConnectionConfig like postgres, oracle, db2_luw and db2i —
no dedicated connection config class or DB_TYPE_MAP entry needed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: beneboy

datamasque/client/models/connection.py

@@ -46,6 +46,7 @@ class DatabaseType(Enum):
     mongodb = "mongodb"
     databricks_lakebase = "databricks_lakebase"
     databricks = "databricks"
+    informix = "informix"
 
 
 class SnowflakeStageLocation(str, Enum):