refactor(dgw): reuse reqwest::Client via OnceLock in AI router

[Copilot]

The AI router was creating a new reqwest::Client for every request. Since configuration is static at runtime, we can cache the client and reuse it.

• Cache HTTP client in a function-local OnceLock, initialized on first use
• Return cloned client (cheap, uses Arc internally)

fn create_client(timeout: Duration) -> reqwest::Client {
    static AI_CLIENT: OnceLock<reqwest::Client> = OnceLock::new();

    AI_CLIENT
        .get_or_init(|| {
            reqwest::Client::builder()
                .timeout(timeout)
                .build()
                .expect("parameters known to be valid only")
        })
        .clone()
}

Original prompt

In devolutions-gateway/src/api/ai.rs, I think it’s better to reuse the reqwest client instead of creating a new one for each request. We could use some form of caching using a OnceLock or LazyLock. Currently, the configuration can’t be changed dynamically, so it’s fine to create once and forget about it. The reqwest::Client can be cloned (it uses Arc internally), so it’s possible to keep the same signature for the create_client function.

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