Fix download capacity accessor on asymmetric links

[thebracket]

Summary

• return the configured downlink capacity from upstream_bandwidth_capacity_download_mbps()
• prevent asymmetric links from using the uplink capacity as the download ceiling

Fixes #1151

Validation

• cargo check -p lqos_python
• cargo test -p lqos_python (7 passed)
• cargo clippy -p lqos_python -- -D warnings
• rustfmt --edition 2024 --check rust/lqos_python/src/lib.rs
• git diff --check