fix: handle bare iterator output annotations

[nightcityblade]

What does this PR address?

Bare iterator and generator return annotations have no type arguments, so indexing get_args() raised IndexError during output-spec inference. This change falls back to Any, matching the existing behavior for an omitted return annotation, and adds regression coverage for all sync and async iterator/generator variants from both typing and collections.abc.

Tests:

• .venv/bin/pytest tests/unit/_bentoml_sdk/test_io_models.py -q (8 passed)
• .venv/bin/pytest tests/unit/bentoml_io/test_decorators.py -q (8 passed)
• .venv/bin/pre-commit run --all-files (passed; Buf hooks skipped because no matching files changed)

Fixes #5625

Before submitting:

• Does the Pull Request follow Conventional Commits specification naming?
• Does the code follow BentoML's code style, pre-commit run -a script has passed?
• Did you read through contribution guidelines and follow development guidelines?
• Did your changes require updates to the documentation? N/A; this only prevents an inference crash and does not change the public API.
• Did you write tests to cover your changes?