Lift default chunk size to 50; bump to 1.2.1

[hemidactylus]

Along with this raise, also some docstring and sample-code improvements came along, esp. around the get_database vs get_database_by_api_endpoint freedom;

also the docstrings have been updated to reflect the latest Data API behaviour with respect to the default projection and the independency of $vector and other special fields from the rest.

This comes with improvements and fixes to the tests:

• splitting in the insert many failure tests to prevent async spilling between functionalities
• adapt to the latest default-projection behaviour also in the "core" testing directory
• adapt to the latest vectorize things from the Data API, namely the expectations on the return value from findEmbeddingProviders call.