Unique ports per Rust server fix vercel dev crashes

Minor Changes

• 69892ba: Fix vercel dev instability with the Rust runtime by allocating a unique free port per dev server instance. Previously the dev server relied on the vercel_runtime crate's fixed default port, which caused intermittent "address already in use" failures (surfacing as Process exited before completing request) when vercel dev restarted the server between requests. The runtime now passes a VERCEL_DEV_PORT, waits for the process to exit during shutdown so the port is released, and reports a clear error on port collisions instead of silently falling back to lambda invocation. The shutdown grace period now also allows the runtime's dev-mode waitUntil drain to complete before force-killing, so background work registered via waitUntil runs as expected under vercel dev.