allow a max_subagents of 0: no subagents for the agent

frontend/packages/swarm-ui/test/max-subagents.test.mjs

@@ -0,0 +1,26 @@
+// The max-subagents field: empty is no cap, 0 is no subagents, and anything
+// that is not a whole number up to the limit is refused. Run by `npm test`.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import {
+  MAX_SUBAGENTS,
+  maxSubagentsValue,
+} from "../src/agent/detail/max-subagents.ts";
+
+test("empty is no cap, and 0 is a cap of none rather than empty", () => {
+  assert.equal(maxSubagentsValue(""), undefined);
+  assert.equal(maxSubagentsValue("  "), undefined);
+  assert.equal(maxSubagentsValue("0"), 0);
+});
+
+test("a whole number up to the limit is taken as typed", () => {
+  assert.equal(maxSubagentsValue("1"), 1);
+  assert.equal(maxSubagentsValue(" 3 "), 3);
+  assert.equal(maxSubagentsValue(String(MAX_SUBAGENTS)), MAX_SUBAGENTS);
+});
+
+test("above the limit, negative, fractional or not a number is refused", () => {
+  for (const text of [String(MAX_SUBAGENTS + 1), "-1", "1.5", "abc", "1e1"]) {
+    assert.equal(maxSubagentsValue(text), null, text);
+  }
+});