fix(plugins): guard legacy zod schemas without toJSONSchema

src/channels/plugins/config-schema.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, it } from "vitest";
+import { z } from "zod";
+import { buildChannelConfigSchema } from "./config-schema.js";
+
+describe("buildChannelConfigSchema", () => {
+  it("builds json schema when toJSONSchema is available", () => {
+    const schema = z.object({ enabled: z.boolean().default(true) });
+    const result = buildChannelConfigSchema(schema);
+    expect(result.schema).toMatchObject({ type: "object" });
+  });
+
+  it("falls back when toJSONSchema is missing (zod v3 plugin compatibility)", () => {
+    const legacySchema = {} as unknown as Parameters<typeof buildChannelConfigSchema>[0];
+    const result = buildChannelConfigSchema(legacySchema);
+    expect(result.schema).toEqual({ type: "object", additionalProperties: true });
+  });
+});