feat: Use Claude Sonnet 4 for code-based slash commands

What does this MR do and why?

Introduce a new feature flag called code_based_slash_commands_claude_4_0_rollout to control whether or not Code-based Slash commands should use Claude Sonnet 4 model.

References

Related Claude 4.0 Sonnet Code-based Slash Commands (#545490 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

When the feature flag is disabled (default), code-based slash commands requests should use Claude 3.7 Sonnet as usual.

Once the flag is enabled, the new Claude Sonnet 4 model is used.

Example:

1. In rails console enable the feature flag for the user you normally use to test features

   Feature.enable(:code_based_slash_commands_claude_4_0_rollout, my_user)

2. On your editor of choice, configure GitLab Duo to point at your local GitLab instance (e.g. VS Code) using the same user from the step above
3. On your editor of choice, try one of /explain, /fix, /refactor or /tests slash commands

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