Update xAI model hosting policy details

Add support for new Grok Code Fast 1 model in xAI section

- Added {% data variables.copilot.copilot_grok_code_fast_1 %} to the list of supported xAI models
- Updated model descriptions to reference both Grok models consistently
- Changed final paragraph to use plural "xAI models" for accuracy
- Maintains existing zero data retention policy coverage for both models

Author: gvasilakiss

content/copilot/reference/ai-models/model-hosting.md

@@ -68,9 +68,13 @@ When using {% data variables.copilot.copilot_gemini %} models, input prompts and
 
 {% data reusables.copilot.grok-promo-period %}
 
-{% data variables.copilot.copilot_grok_code %} is hosted by xAI when used in {% data variables.product.prodname_copilot %}.
+Used for:
+* {% data variables.copilot.copilot_grok_code %}
+* {% data variables.copilot.copilot_grok_code_fast_1 %}
+
+{% data variables.copilot.copilot_grok_code %} and {% data variables.copilot.copilot_grok_code_fast_1 %} are hosted by xAI when used in {% data variables.product.prodname_copilot %}.
 
-xAI operates {% data variables.copilot.copilot_grok_code %} in {% data variables.product.prodname_copilot %} under a zero data retention API policy. This means xAI commits that user content (both inputs sent to the model and outputs generated by the model):
+xAI operates {% data variables.copilot.copilot_grok_code %} and {% data variables.copilot.copilot_grok_code_fast_1 %} in {% data variables.product.prodname_copilot %} under a zero data retention API policy. This means xAI commits that user content (both inputs sent to the model and outputs generated by the model):
 
 Will **not** be:
 * Logged for any purpose, including human review
@@ -81,4 +85,4 @@ Will **only**:
 * Exist temporarily in RAM for the minimum time required to process and respond to each request
 * Be immediately deleted from memory once the response is delivered
 
-When using xAI, input prompts and output completions continue to run through {% data variables.product.prodname_copilot %}'s content filters for public code matching, when applied, along with those for harmful or offensive content.
+When using xAI models, input prompts and output completions continue to run through {% data variables.product.prodname_copilot %}'s content filters for public code matching, when applied, along with those for harmful or offensive content.