refactor(test): dedupe agent and discord test fixtures

src/agents/test-helpers/model-fallback-config-fixture.ts

@@ -0,0 +1,15 @@
+import type { OpenClawConfig } from "../../config/config.js";
+
+export function makeModelFallbackCfg(overrides: Partial<OpenClawConfig> = {}): OpenClawConfig {
+  return {
+    agents: {
+      defaults: {
+        model: {
+          primary: "openai/gpt-4.1-mini",
+          fallbacks: ["anthropic/claude-haiku-3-5"],
+        },
+      },
+    },
+    ...overrides,
+  } as OpenClawConfig;
+}