refactor(agent): drop redundant str() wrapping around custom_provider_slug args

Gate C11 r4 quality: custom_provider_slug already coerces with
str(value or "").strip(), so the caller-side wrapping was noise.
No behaviour change.

agent/reasoning_promotion.py

@@ -57,7 +57,7 @@ def answer_in_reasoning_capability(agent: Any) -> bool:
     ids = {custom_provider_slug(p) for p in (provider, requested) if p.startswith("custom:")}
     entries = list(_entries_for_route(base_url, custom_providers, None))
     live = [e for e in entries
-            if custom_provider_slug(str(e.get("name") or ""), str(e.get("provider_key") or "")) in ids]
+            if custom_provider_slug(e.get("name"), e.get("provider_key")) in ids]
     flags = {v for e in live or entries
              if isinstance(v := (e.get("capabilities") or {}).get(_ANSWER_IN_REASONING_CAPABILITY), bool)}
     if flags: