fix(xai): keep image_generation on grok-4.6+ conversation requests

normalizeXAITool still strips Codex hosted image tools on older Grok
conversation models. grok-4.6 and later accept xAI native Imagine
tool, so keep client-supplied image_generation there and rewrite a
forced choice into allowed_tools. grok-4.20-* stays on the old strip
because that product line is not comparable to grok-4.6.

Closes: #5173