Add Rust example for delivery/stream binding

[George-iam]

All examples currently support Python, TypeScript, Go, Java, and .NET. A Rust implementation would expand coverage to a popular systems programming community.

What needs to happen:

• Add examples/delivery/stream/rust/agent.rs with SSE stream listener
• Add Cargo.toml in lang/rust/
• Follow the existing pattern: listen → process → resume

Good first issue for Rust developers wanting to try AXME.