[BUG] Fix missing raise before TypeError in Pipeline._check_steps

[Sanchit2662]

Description

--- I found a missing raise keyword in the pipeline validation logic that was silently allowing invalid configurations to pass through.

In skpro/regression/compose/_pipeline.py, line 111 was creating a TypeError but never raising it. This meant if someone built a pipeline with a transformer as the last step (when allow_postproc=False), the validation would just skip over the error instead of failing.

Fix

Added raise before the TypeError on line 111. Now invalid pipelines properly raise an exception instead of silently succeeding and breaking later during fit/predict.

It's a one-word fix that aligns with the two other validation checks in the same method which already use raise correctly. Pretty straightforward catch.

[Sanchit2662]

Hi @fkiraly , please look review the changes.