xAI rejects minLength, maxLength, minItems, maxItems, minContains, and
maxContains in tool schemas with a 502 error instead of ignoring them.
This causes all requests to fail when any tool definition includes these
validation-constraint keywords (e.g. sessions_spawn uses maxLength and
maxItems on its attachment fields).
Add stripXaiUnsupportedKeywords() in schema/clean-for-xai.ts, mirroring
the existing cleanSchemaForGemini() pattern. Apply it in normalizeToolParameters()
when the provider is xai directly, or openrouter with an x-ai/* model id.
Fixes tool calls for x-ai/grok-* models both direct and via OpenRouter.
00347bda75