Honor call=False in import_executor - #650

Summary

import_executor() documents a call flag but ignores it: if the value is callable, it is always called with the remaining kwargs. So import_executor("gpu", call=False) returns an executor instead of the factory, and a factory with required arguments raises TypeError.

With this change, call=False returns the value without calling it, and the docstring notes that kwargs are unused in that case. The default call=True path is unchanged. Executor instances are still returned as-is with either flag value, and the return annotation is still Executor.

Tests

Local results (Linux, Python 3.11):

• New tests: five cases using a real temporary executors file. Two are call=False regressions, one of them using a factory with a required argument. The other three are controls for executor instances (with both flag values) and for the default factory call.
• test_base.py: 2 failed / 19 passed before the fix, 21 passed after.
• Full suite: 1717 passed, 1 deselected, 1 xfailed. After the summary, Ray prints an ignored atexit ray.dag import error; unmodified main prints the same message.
• ruff check and ruff format --check pass.