fix(io_models): guard against IndexError for bare iterator return annotations

[devteamaegis]

What's broken

IODescriptor.from_output() raises IndexError: tuple index out of range whenever a service method is annotated with an unparameterized iterator type (t.Iterator, t.Generator, t.AsyncIterator, t.AsyncGenerator, or their collections.abc equivalents). Service startup crashes before any request is served.

Why it happens

is_iterator_type() correctly returns True for bare generics, but get_args() returns an empty tuple () for types with no type parameters. Line 435 unconditionally indexes position 0 without a bounds check. The surrounding try/except only catches ValueError and TypeError, so the IndexError escapes.

Fix

Added a one-line guard: args = get_args(return_annotation); return_annotation = args[0] if args else t.Any. When no type argument is present the descriptor defaults to t.Any, which is the same behaviour used when the return annotation is absent entirely.

Test

Added tests/unit/_bentoml_sdk/test_io_models.py::test_from_output_bare_iterator_does_not_raise which calls IODescriptor.from_output() with a -> t.Iterator annotated function and asserts it returns without raising.

Fixes #5625