Rust SDK

Add Bench using https://docs.usebench.ai/sdk/rust and my Bench SDK setup prompt.
Use its installation source, endpoint and server-side key. Start with metadata
only in staging. Wrap a real async operation and its tools, passing TraceContext
explicitly to child operations. Flush a synthetic interaction and confirm receipt
in Bench. Show changed files and remaining setup steps.

use trybench_sdk::{Bench, Options, SpanInput};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut options = Options::new(
        std::env::var("BENCH_API_KEY")?,
        std::env::var("BENCH_REPOSITORY")?,
        std::env::var("BENCH_BRANCH")?,
    );
    options.endpoint = std::env::var("BENCH_API_BASE_URL")?;
    options.environment = Some("staging".into());
    options.system_name = Some("Support agent".into());
    let bench = Bench::new(options)?;
    let result = bench.trace(
        None,
        SpanInput::new("connection-check").kind("AGENT"),
        |_context| async { Ok::<_, String>(true) },
    ).await;
    bench.shutdown().await;
    result?;
    Ok(())
}