test(apiproxy): follow the defaultModelSelection rename in the preset spec

The agent-default-model refactor renamed ApiProxyDefaults.defaultTarget to
defaultModelSelection; this stack-owned spec merged without textual
conflict and kept the old field, so its harness built a defaults object
whose selection thunk was undefined and every create answered internal.

packages/host/apiproxy/tests/api-proxy-agent-preset.spec.ts

@@ -109,7 +109,7 @@ async function harness(presets?: readonly string[], persistence?: unknown) {
   }
   ctx.agents.setFactory(factory)
   const api = createApiProxy(ctx, {
-    defaultTarget: () => ({ provider: 'test', model: 'test-model' }),
+    defaultModelSelection: () => ({ provider: 'test', model: 'test-model' }),
     cwd,
     workspaceRoot: cwd,
   })