fix: InputSocket.__post_init__ crashes with IndexError on unsubscripted Variadic type

[devteamaegis]

In InputSocket.__post_init__, line 100 calls get_args(get_args(self.type)[0])[0] to unwrap the inner type from a Variadic / GreedyVariadic annotation. This assumes the wrapped type is always a subscripted generic (e.g. Iterable[int]), but if a developer annotates a run-method parameter with a bare Annotated[Iterable, HAYSTACK_VARIADIC_ANNOTATION] — using Iterable without a type argument — then get_args(Iterable) returns an empty tuple (), and the unconditional [0] index raises IndexError: tuple index out of range.

This crash surfaces at component instantiation time (not import time), so it bypasses static type checking and takes down the whole component construction.

Reproduction:

from typing import Annotated
from collections.abc import Iterable
from haystack import component
from haystack.core.component.types import HAYSTACK_VARIADIC_ANNOTATION

@component
class BadJoiner:
    @component.output_types(result=str)
    def run(self, inputs: Annotated[Iterable, HAYSTACK_VARIADIC_ANNOTATION]):
        return {"result": ""}

j = BadJoiner()  # IndexError: tuple index out of range

Expected: a clear ComponentError explaining that a type argument is required, e.g. Variadic[int].