Reject misplaced empty `runtime_kwargs`

Treat `runtime_kwargs` as provided whenever it is not `None`.
Previously an empty dict bypassed placement validation and was
silently ignored when `an` or `portal` selected an existing runtime.

Reject both placement modes before actor startup for empty and
configured runtime kwargs while preserving empty-dict use when
`to_actor.run()` owns its private runtime.

Caught-during: review remediation
Found-via: `/code-review` P3 option-validation finding

Review: PR #481 (opencode)
https://github.com/goodboy/tractor/pull/481#pullrequestreview-4956692120

Prompt-IO: ai/prompt-io/opencode/20260819T020757Z_b38efed7_prompt_io.md

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

ai/prompt-io/opencode/20260819T020757Z_b38efed7_prompt_io.md

@@ -0,0 +1,34 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+session: ses_3e4c90d3eafeqHEtRYSIHgHhpA
+timestamp: 2026-08-19T02:07:57Z
+git_ref: b38efed7
+scope: code
+substantive: true
+raw_file: 20260819T020757Z_b38efed7_prompt_io.raw.md
+---
+
+## Prompt
+
+Resolve the remaining P3 review finding before landing PR #481:
+`runtime_kwargs={}` must not be silently accepted alongside either
+`an=` or `portal=` merely because the dict is falsey. Keep this as its
+own final review-remediation commit.
+
+## Response summary
+
+Treat any non-`None` `runtime_kwargs` value as provided when validating
+placement. Cover both placement APIs with empty and configured dicts,
+proving the error is raised locally before actor startup.
+
+## Files changed
+
+- `tractor/to_actor/_api.py` - validate option presence explicitly.
+- `tests/test_to_actor.py` - cover four invalid option combinations.
+
+## Human edits
+
+No direct line edits. The human accepted the P3 finding, required it to
+remain separate from the five P2 behavioral commits and prioritized it
+before the final PR #484 integration rebase and PR #481 landing steps.

ai/prompt-io/opencode/20260819T020757Z_b38efed7_prompt_io.raw.md

@@ -0,0 +1,25 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+timestamp: 2026-08-19T02:07:57Z
+git_ref: b38efed7
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+Fix the final PR #481 review finding: `runtime_kwargs` is mutually
+exclusive with both caller placement options whenever it is provided,
+including an empty dict.
+
+> `git diff HEAD~1..HEAD -- tractor/to_actor/_api.py tests/test_to_actor.py`
+
+Use an explicit `is not None` check rather than dict truthiness. Expand
+the validation regression across `an=` and `portal=`, each with empty
+and configured runtime kwargs, so every invalid combination fails
+before actor runtime startup.
+
+Verification:
+
+- Trio/TCP: `23 passed`
+- Trio/UDS: `23 passed`
+- `mp_spawn`/TCP: `23 passed`
+- Ruff and `git diff --check`: clean

tests/test_to_actor.py

@@ -511,12 +511,30 @@ def test_rejects_portal_and_an_combo():
         )
 
 
-def test_rejects_runtime_kwargs_with_placement():
+@pytest.mark.parametrize(
+    'placement',
+    ['an', 'portal'],
+)
+@pytest.mark.parametrize(
+    'runtime_kwargs',
+    [
+        {},
+        {'loglevel': 'cancel'},
+    ],
+    ids=['empty', 'configured'],
+)
+def test_rejects_runtime_kwargs_with_placement(
+    placement: str,
+    runtime_kwargs: dict,
+):
     '''
     `runtime_kwargs` only applies when the call opens
     its own private actor-nursery; passing it alongside
     a placement opt is an error, never silently
-    ignored.
+    ignored. In particular, an empty dict still means the
+    caller provided this mutually exclusive option; testing
+    both placement modes prevents truthiness checks from
+    accepting it before any actor runtime is started.
 
     '''
     with pytest.raises(ValueError):
@@ -525,10 +543,10 @@ def test_rejects_runtime_kwargs_with_placement():
                 to_actor.run,
                 add_one,
                 1,
-                an=object(),
-                runtime_kwargs=dict(
-                    loglevel='cancel',
-                ),
+                **{
+                    placement: object(),
+                    'runtime_kwargs': runtime_kwargs,
+                },
             )
         )
 

tractor/to_actor/_api.py

@@ -295,7 +295,7 @@ async def run(
     fn, args, kwargs = _normalize_call(fn, args)
 
     if (
-        runtime_kwargs
+        runtime_kwargs is not None
         and
         (
             an is not None