fix: optional database dependencies work without each other now

pydase_service_base/database/__init__.py

@@ -1,4 +1,33 @@
-from .influxdb_session import InfluxDBSession
+class OptionalDependencyError(Exception):
-from .postgres_session import PostgresDatabaseSession
+    """Exception raised when an optional dependency is not installed."""
+
+
+try:
+    import influxdb_client  # type: ignore # noqa
+
+    from .influxdb_session import InfluxDBSession  # type: ignore
+except ImportError:
+
+    class InfluxDBSession:  # type: ignore
+        def __init__(self) -> None:
+            raise OptionalDependencyError(
+                "InfluxDBSession requires the 'influxdbv2' extra. "
+                "Please refer to https://gitlab.phys.ethz.ch/tiqi-projects/qchub/icon-services/pydase_service_base."
+            )
+
+
+try:
+    import sqlmodel  # noqa
+
+    from .postgres_session import PostgresDatabaseSession  # type: ignore
+except ImportError:
+
+    class PostgresDatabaseSession:  # type: ignore
+        def __init__(self) -> None:
+            raise OptionalDependencyError(
+                "PostgresDatabaseSession requires the 'postgresql' extra. "
+                "Please refer to https://gitlab.phys.ethz.ch/tiqi-projects/qchub/icon-services/pydase_service_base."
+            )
+
 
 __all__ = ["InfluxDBSession", "PostgresDatabaseSession"]