QA: keep explicit Matrix model overrides lazy

Avoid loading the qa-lab runtime model-default seam when the Matrix runner was
already given both primary and alternate model refs explicitly.

Behavior stays the same, but the helper becomes easier to read and avoids an
unnecessary runtime dependency on the fully-explicit path.

extensions/qa-matrix/src/runners/contract/model-selection.test.ts

@@ -45,6 +45,7 @@ describe("matrix qa model selection", () => {
       primaryModel: "custom-primary",
       alternateModel: "custom-alt",
     });
+    expect(loadQaLabRuntimeModule).not.toHaveBeenCalled();
     expect(defaultQaRuntimeModelForMode).not.toHaveBeenCalled();
   });
 });

extensions/qa-matrix/src/runners/contract/model-selection.ts

@@ -13,14 +13,21 @@ export function resolveMatrixQaModels(params: {
   alternateModel?: string;
 }): ResolvedMatrixQaModels {
   const providerMode = normalizeQaProviderMode(params.providerMode ?? "live-frontier");
+  const primaryModel = params.primaryModel?.trim();
+  const alternateModel = params.alternateModel?.trim();
+  if (primaryModel && alternateModel) {
+    return {
+      providerMode,
+      primaryModel,
+      alternateModel,
+    };
+  }
+
   const qaLabRuntime = loadQaLabRuntimeModule();
   return {
     providerMode,
-    primaryModel:
-      params.primaryModel?.trim() ||
-      qaLabRuntime.defaultQaRuntimeModelForMode(providerMode),
+    primaryModel: primaryModel || qaLabRuntime.defaultQaRuntimeModelForMode(providerMode),
     alternateModel:
-      params.alternateModel?.trim() ||
-      qaLabRuntime.defaultQaRuntimeModelForMode(providerMode, { alternate: true }),
+      alternateModel || qaLabRuntime.defaultQaRuntimeModelForMode(providerMode, { alternate: true }),
   };
 }