fix(python): return None from unit jobs (#3999)

## Problem

Generic job result propagation exposed the PyO3 representation of Rust's
unit value as `()` in Python. Unit jobs therefore returned an empty
tuple instead of `None`, breaking the documented `Job.wait()` contract
and the Python doctest workflow.

## Behavior

Unit job completion now converts explicitly to Python `None`. Typed job
results continue to pass through unchanged, with synchronous and
asynchronous regression coverage.

python/src/job.rs

@@ -93,7 +93,7 @@ impl Job {
         let inner = self_.inner.clone();
         future_into_py(self_.py(), async move {
             inner.wait().await.infer_error()?;
-            Ok(())
+            Ok(None::<()>)
         })
     }