[Bug]: Many snippets in snippets.py are untested

### What happened?

See #22042

The examples in snippets.py should be tested continuously to make sure we're not advertising flawed code. Some of the snippets are tested in snippets_test.py, e.g.: https://github.com/apache/beam/blob/e86456ea71b1203b7158496e4653541809415225/sdks/python/apache_beam/examples/snippets/snippets_test.py#L627-L633

but it looks like many others (like `sdf_basic_example`) are not. We should add tests exercising all of these examples.

### Issue Priority

Priority: 2

### Issue Component

Component: examples-python