fix(intern-decision-serve): 0.1.2 treats empty/null images as absent

Audit finding on 0.1.1: a Jev client that always sends an images array with no
images was rejected for nothing. Only a non-empty value is 422 now. Also aligns
the contract's response example with the wire (model is the name@revision
string, not an object).

Re-accepted live: images []/null -> 200, ["a.png"] -> 422; JevBench all 202/231,
hard 83/111, 0 changed rows across the bench's r1..r4 (924).

services/intern-decision-serve/src/intern_decision_serve/app.py

@@ -78,7 +78,9 @@ class SystemOneBody(BaseModel):
 
     @property
     def images_present(self) -> bool:
-        return "images" in (self.model_extra or {})
+        """A NON-EMPTY images value only: [] and null count as absent (audit 2026-09-30 — a
+        client that always sends the field must not be rejected for nothing)."""
+        return bool((self.model_extra or {}).get("images"))
 
 
 class ApiError(Exception):