Fix FlowAppendView support for 'once' flag by forcing batch read

[liamperritt]

Implementing a fix for appending views using the 'once' flag by ensuring the returned value is a batch DataFrame, not a streaming DataFrame.

As outlined in the append_flow documentation:

Using once=True changes the flow: the return value must be a batch DataFrame in this case, not a streaming DataFrame.

Currently, configuring the 'once' flag with a batch data source fails with:
View is not a streaming view and must be referenced using read.

I've tested this fix manually, and it resolves the issue: it successfully appends a one-time batch flow into the target streaming table, as expected.