fix: Fix "Connection is closed" error when using browser authenticati…

…on (#257)

The purpose of this PR is to solve the issue reported
[here](#225).

## Root cause:

There was one usage of `engine.connect` outside of a context manager (in
`create_engine`), causing the connector to leave one connection "lying
around".

I did not dive too deep into the internals of the connector nor focused
too much in understanding why is it different for the browser
authentication mechanism, but mostly in ensuring all calls to
`engine.connect()` were done within a context manager, ensuring SQL
alchemy does its job in closing all connections when they should be
closed.

## Implementation details:

- Use a context manager when checking if the database exists (in the
`create_engine` ) method.

target_snowflake/connector.py

@@ -192,11 +192,11 @@ def create_engine(self) -> Engine:
             connect_args=connect_args,
             echo=False,
         )
-        connection = engine.connect()
-        db_names = [db[1] for db in connection.execute(text("SHOW DATABASES;")).fetchall()]
-        if self.config["database"] not in db_names:
-            msg = f"Database '{self.config['database']}' does not exist or the user/role doesn't have access to it."
-            raise Exception(msg)  # noqa: TRY002
+        with engine.connect() as conn:
+            db_names = [db[1] for db in conn.execute(text("SHOW DATABASES;")).fetchall()]
+            if self.config["database"] not in db_names:
+                msg = f"Database '{self.config['database']}' does not exist or the user/role doesn't have access to it."
+                raise Exception(msg)  # noqa: TRY002
         return engine
 
     def prepare_column(