Remove ai_gateway_multi_default_models and duo_workflow_provider_stickiness feature flags

What does this MR do and why?

Removes two fully-rolled-out feature flags and all their references in application code and specs:

ai_gateway_multi_default_models

• Deleted the YAML definition (config/feature_flags/gitlab_com_derisk/ai_gateway_multi_default_models.yml)
• Removed Gitlab::AiGateway.push_feature_flag(:ai_gateway_multi_default_models, ...) calls from:
  • ee/app/services/ai/model_selection/fetch_model_definitions_service.rb
  • ee/lib/gitlab/llm/completions/chat.rb
  • ee/lib/api/code_suggestions.rb
  • ee/lib/api/helpers/duo_workflow_helpers.rb
• Simplified the cache key in FetchModelDefinitionsService to use RESPONSE_CACHE_NAME directly (removed the flag-state suffix per @alejandro's instruction)

duo_workflow_provider_stickiness

• Deleted the YAML definition (ee/config/feature_flags/gitlab_com_derisk/duo_workflow_provider_stickiness.yml)
• Made provider stickiness (reusing stored model_metadata_json from a workflow) the unconditional default in ee/lib/api/ai/duo_workflows/workflows.rb

All specs updated to reflect the now-default behavior.

References

• #601113
• Resolves #627607