Split token display for xAI

Extends the split input/output token display (introduced in #46829 for OpenAI) to all xAI models.

Instead of the combined 48k / 1M token counter, xAI models now show:

• ↑ input tokens used / input token limit
• ↓ output tokens used / output token limit

Before

After

Changes

• x_ai.rs — Override supports_split_token_display() to return true on XAiLanguageModel. All built-in Grok models already implement max_output_tokens(), so no additional plumbing was needed.
• cloud.rs — Add XAi to the matches! pattern in CloudLanguageModel::supports_split_token_display() so cloud-routed xAI models also get the split display.

Tests

• test_xai_supports_split_token_display — Verifies all built-in Grok model variants return true for split token display.
• test_xai_models_have_max_output_tokens — Validates all built-in Grok models report max_output_tokens that is Some, positive, and less than max_token_count (required for the UI to compute the input token limit).
• test_split_token_display_supported_providers — Confirms the cloud provider match pattern includes OpenAi and XAi while excluding Anthropic and Google.

Release Notes:

• Changed the display of tokens for xAI models to reflect the input/output limits.