[ENHANCEMENT] Add Grok 4 Fast to xAI Provider

[0Shard]

Problem (one or two sentences)

Grok 4 Fast is not available in the model selection for the xAI provider, making it impossible to use this model directly with an xAI account.

Context (who is affected and when)

This affects all users who have access to Grok 4 Fast through the xAI platform and wish to use it within Roo Code.

Desired behavior (conceptual, not technical)

Users should be able to select "Grok 4 Fast" from the list of available models when using the xAI provider.

Constraints / preferences (optional)

No response

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Roo Code Task Links (optional)

No response

Acceptance criteria (optional)

No response

Proposed approach (optional)

No response

Trade-offs / risks (optional)

No response

[roomote]

I'm exploring a potential fix for adding Grok 4 Fast to the xAI provider and will open a PR to share progress.

[roomote]

Opened PR #8476 with my attempt at addressing this issue. All CI checks have passed successfully. The implementation adds the grok-4-fast model to the xAI provider with appropriate configuration and test coverage. Feedback welcome!

PR: #8476