Assert `to_actor.run()` runtime lifecycle

The implicit-runtime test verified the caller started outside Tractor
but did not prove that the target ran inside an actor or that the
private runtime was gone when the call returned.

Assert an active actor inside the shared remote target and assert the
caller has no current actor again after the one-shot call completes.

Review: PR #481 (goodboy)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-5012942328

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

tests/test_to_actor.py

@@ -33,6 +33,13 @@ from ._helpers import (
 async def add_one(
     n: int,
 ) -> int:
+    '''
+    Increment within an active actor runtime.
+
+    '''
+    assert tractor.current_actor(
+        err_on_no_runtime=False,
+    ) is not None
     return n + 1
 
 
@@ -125,7 +132,9 @@ def test_one_shot_boots_implicit_runtime(
     '''
     Outside any actor-runtime `to_actor.run()` boots one
     implicitly (just like bare `open_nursery()` usage)
-    configured via pass-through `runtime_kwargs`.
+    configured via pass-through `runtime_kwargs`. The remote target
+    asserts its runtime exists; the caller then verifies the private
+    runtime is fully torn down before `to_actor.run()` returns.
 
     '''
     async def main() -> None:
@@ -142,6 +151,9 @@ def test_one_shot_boots_implicit_runtime(
             ),
         )
         assert result == 42
+        assert tractor.current_actor(
+            err_on_no_runtime=False,
+        ) is None
 
     trio.run(main)