Rust v4.0.21 parity fixes for the Python SDK.\n\nHighlights:\n- Tightens SWQOS sender/provider request handling and signature validation.\n- Preserves provider-specific options such as Helius swqos_only and Astralane/Node1 transports.\n- Aligns multi-SWQOS durable nonce and with_tip=false behavior.\n- Updates tests and README parity notes.