fix(api-nodes): enable 2 more pylint rules, removed non needed code

[bigcat88]

1. We can safely enable the import-outside-toplevel rule, since there is no reason for API nodes not to import all required modules at the top of the file. If there is ever a strong need to import something elsewhere, we can mark that line with # noqa.

2. After enabling the inconsistent-return-statements rule, I reviewed the validate_input_media function. It sometimes returns None, but overall does nothing useful. It can return a string in case of failed validation, but its result is never checked anywhere. When testing the API, I found that rewriting it to raise an error on validation would break some existing workflows, because the Moonvalley API successfully processes videos with non-odd dimensions (so the check itself is invalid if we enforce it). I also could not find any cases where with_frame_conditioning is required - maybe it was intended for a future video2video implementation that does not yet exist. For now, I suggest removing this unused function completely. If we need this functionality later, we can re-implement it in validation_utils.py so it can benefit other nodes and be easy to find/test.

Test that moving import does not break Gemini node:

Test to prove that dimension check for Moonvalley node is incorrect if we enable it:

moonvalley.mp4

[bigcat88]

+label: Core

[Kosinkadink]

Gotcha, since you've tested the conditions that the checks were looking for, looks good. I'll merge.