GHSA-wrq8-fcv5-8hvp: Coder: Route hijacking through lack of validation of agent-supplied AllowedIPs in tailnet coordinator

Coder's tailnet coordinator (the server managing a private network) didn't validate that `AllowedIPs` (IP address ranges an agent claims to serve) matched the agent's identity, even though it did validate the agent's regular addresses. This let a malicious authenticated agent claim another agent's IP address and intercept traffic meant for that agent, such as web terminal sessions. Exploiting this required an authenticated user with a modified agent binary.