fix(cli): restore fallback for unknown kernel language/type in kernels_pull

[sridipbasu]

Summary

This fixes a bug in kernels_pull where the existing fallback for unknown kernel language/type combinations was never reached.

If the language/type doesn't match one of the known extension mappings, extension remains None. The code then attempts to build the filename using:

file_name = blob.slug + extension

which raises a TypeError before the fallback logic can execute.

Fix

Instead of concatenating immediately, the code now checks whether an extension was resolved first. If not, it allows the existing fallback to handle the case and use script.py.

This doesn't change the behavior for any of the currently supported language/type mappings.

Tests

Added regression tests covering:

• unknown language
• empty language
• empty kernel type
• unknown kernel type

Also verified that existing mappings continue to work correctly for supported kernel types.

Validation

• ✅ pytest tests/unit
• ✅ black --check .
• ✅ mypy src/kaggle tests