Fix error in FeatureSetting decorator when model_definitions is nil

What does this MR do and why?

aiChatAvailableModels query, which returns available models for user-level model selection, returns an Internal Server Error for offline licenses due to this line. FetchModelDefinitionsService returns nil for offline licenses, so the decorator's feature_data method calls model_definitions['unit_primitives'] on nil, raising a NoMethodError.

References

Resolves [IDE] aiChatAvailableModels query from LS retur... (#596569 - closed)

Screenshots or screen recordings

Offline license:

Before	After

No regressions on user-level model selection

Web	IDE

How to set up and validate locally

Prerequisites:

• Offline license with DAP self-hosted add-on and an online license, you can use the ones here.
• GitLab Workflow extension installed in your VSCode
• Setup GitLab authentication with your instance:
  • Open the command palette (Cmd+Shift+P) > type in GitLab: Authenticate
  • Manually enter instance URL > Enter your instance URL
  • Select Create a token
  • Ensure the account you just authenticated with is selected.

Steps:

• Run GDK on SM mode: GITLAB_SIMULATE_SAAS=0 gdk start
• Clone a project and open in VSCode
• Open agentic chat
• Observe no error message returned

MR acceptance checklist

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