Resolve "Update Code Creation Slash Commands to Claude 3.7 Sonnet"

What does this MR do and why?

Adds code_creation_slash_commands_claude_3_7 feature flag to enable using Claude 3.7 Sonnet for code creation slash commands (/tests, /fix, /explain, /refactor)

Requires AIGW MR gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2050 (merged) to be merged

References

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MR acceptance checklist

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Before	After

How to set up and validate locally

1. Feature.enable(:code_creation_slash_commands_claude_3_7, YOUR_USER)
2. Make sure your chat is connected to your local GDK
3. Use /tests, /fix, /refactor or /explain in Duo Chat
4. Check the AI Gateway logs and make sure the Request to LLM contains 'model': 'claude-3-7-sonnet-20250219'

Related to #521410 (closed)