[SPARK-56247][PYTHON] Fix the fall back behavior and type hint of inheritable_thread_target

[gaogaotiantian]

What changes were proposed in this pull request?

• Removed None from the possible input type hint
• When f is a session, return a decorator instead of f if pinned thread mode is off

Why are the changes needed?

Th input f can't be None - we have no logic to deal with it. Also as a decorator, the way it works is to deal with

@inheritable_thread_target()
def f():
    ...

but we need special logic for this - we don't have that.

When pinned-thread mode is off (legacy, we don't care much, but still nice to have it right), we need @inheritable_thread_target(session) to be a no-op, instead of return session directly, which will raise an exception when we do session(f).

Does this PR introduce any user-facing change?

Not really, we are fixing an unexpected behavior.

How was this patch tested?

CI should pass. We don't really care too much about the legacy behavior though.

Was this patch authored or co-authored using generative AI tooling?

No.

[zhengruifeng]

merged to master