feat(gateway): validate streaming passthrough

[kev1n77]

Summary

• add an Axum/Reqwest local AI Gateway spike with streamed request and response bodies
• preserve method, path, query, headers, status and cancellation while removing hop-by-hop headers
• expose a runnable gateway-spike binary and document its environment variables
• update the progress tracker after the streaming acceptance tests passed

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings