The dns toggle button called fetch(..., { method: "PUT" }) but
src/app/api/tools/agent-bridge/agents/[id]/dns/route.ts only exports
POST, so Next.js auto-returned 405 on every Start/Stop DNS click.
Fixes the frontend caller to match the documented POST contract
(docs/frameworks/AGENTBRIDGE.md:490) already covered by
tests/unit/agent-bridge-dns-route-validation.test.ts.
Adds a regression test asserting the fetch call uses method: POST.
