Function block_on_async 

pub fn block_on_async<F, T>(f: F) -> T
where
    F: Future<Output = T> + Send + 'static,
    T: Send + 'static,

Run an async block synchronously from sync code.

Used for testnet-only initialization (funding test accounts in engine builder). Handles both multi-threaded and single-threaded tokio runtimes:

• Multi-threaded: uses block_in_place (efficient, does not block other tasks)
• Single-threaded: spawns a thread with new runtime (avoids deadlock)
• No runtime: creates a temporary runtime

NOTE: This is only called when modes.testnet_mode=true. In production, the code path that uses this is skipped entirely.