Fix model fallback crash on undefined provider/model (#946)

src/agents/model-fallback.test.ts

@@ -213,6 +213,34 @@ describe("runWithModelFallback", () => {
     expect(calls).toEqual([{ provider: "anthropic", model: "claude-opus-4-5" }]);
   });
 
+  it("defaults provider/model when missing (regression #946)", async () => {
+    const cfg = makeCfg({
+      agents: {
+        defaults: {
+          model: {
+            primary: "openai/gpt-4.1-mini",
+            fallbacks: [],
+          },
+        },
+      },
+    });
+
+    const calls: Array<{ provider: string; model: string }> = [];
+
+    const result = await runWithModelFallback({
+      cfg,
+      provider: undefined as unknown as string,
+      model: undefined as unknown as string,
+      run: async (provider, model) => {
+        calls.push({ provider, model });
+        return "ok";
+      },
+    });
+
+    expect(result.result).toBe("ok");
+    expect(calls).toEqual([{ provider: "openai", model: "gpt-4.1-mini" }]);
+  });
+
   it("falls back on missing API key errors", async () => {
     const cfg = makeCfg();
     const run = vi