Reasoning effort parameter handling fixed

Patch Changes

• 8e006de: fix: send reasoning effort none to the xAI API when the top-level reasoning: 'none' option is set
• 8e006de: fix: omit the reasoning effort parameter and emit an unsupported warning when the top-level reasoning option is used with xAI models that reject it (grok-4.20-reasoning, grok-4.20-non-reasoning, and dated variants)