Rename the createServiceToken service to "ai-gateway" (#56035)

`getServiceToken`'s service string becomes `ai-gateway`, matching the gateway's hostname `ai-gateway.convex.dev`. Two gates read that string, and both move together.

```
V8 / HTTP action                    Node action
  getServiceToken("ai-gateway")       getServiceToken("ai-gateway")
        │  syscall                           │  syscall
        ▼                                    ▼
  crates/isolate                       node-executor
  async_syscall.rs                     src/syscalls.ts
    enum Service                         createServiceTokenSchema
    rename "ai-gateway"                  z.literal("ai-gateway")
```

Missing either gate breaks one runtime and not the other, so tests pin both the new name and the rejection of the old one.

The syscall name `1.0/createServiceToken` is untouched. Old bundles send it, so it stays frozen; only the argument's value changes.

GitOrigin-RevId: ece1cec4d2a15989cfb8b9bdd93ed4b877b9a49a