Add thinking_mode and feature_config handling

Introduce a thinking_mode option and build a conditional feature_config payload for Qwen requests: when reasoning is enabled the config includes auto_thinking, thinking_mode, thinking_enabled, output_schema, research_mode and auto_search; otherwise it provides a minimal config with thinking_budget. Also include minor cleanup and formatting/PEP8 fixes (spacing around defaults, blank-line adjustments, and reflowed long request lines) to improve readability.