Raise a PipelineWrapperError if run_api or run_api_async have no return type

[mpangrazzi]

When you have something like:

from haystack import Pipeline
from hayhooks import BasePipelineWrapper


class PipelineWrapper(BasePipelineWrapper):
    def setup(self):
        self.pipeline = Pipeline()

    def run_api(self, test_param: str):
        return f"Dummy result with {test_param}"

So no defined return type for run_api. Currently this is unhandled in Hayhooks (deploy fails since it cannot dynamically create a response model).

The idea is instead to return an error with this message: Pipeline wrapper is missing a return type for 'run_api' method.

Note that in FastAPI it's not mandatory to have a Pydantic model for response, but generally it's highly recommended and will improve DX and docs.

[vblagoje]

LGTM