Renames CodeSuggestions namespace terminologies and updates test coverage

ee/lib/code_suggestions/instructions_extractor.rb

ee/lib/code_suggestions/programming_language.rb

ee/lib/code_suggestions/prompts/base.rb

...pts/code_completion/ai_gateway_code_completion_message.rb

...uggestions/prompts/code_generation/ai_gateway_messages.rb

...rompts/code_generation/ai_gateway_self_hosted_messages.rb

...suggestions/prompts/code_generation/anthropic_messages.rb

ee/lib/code_suggestions/task_factory.rb

...lab/llm/vertex_ai/model_configurations/code_completion.rb

ee/spec/features/custom_models/code_suggestions_spec.rb

ee/spec/lib/code_suggestions/instructions_extractor_spec.rb

ee/spec/lib/code_suggestions/programming_language_spec.rb

...ode_completion/ai_gateway_code_completion_message_spec.rb

...tions/prompts/code_generation/ai_gateway_messages_spec.rb

...s/code_generation/ai_gateway_self_hosted_messages_spec.rb

...ons/prompts/code_generation/ai_gateway_shared_examples.rb

ee/spec/lib/code_suggestions/task_factory_spec.rb

ee/spec/lib/code_suggestions/tasks/code_completion_spec.rb

ee/spec/lib/code_suggestions/tasks/code_generation_spec.rb

ee/spec/lib/gitlab/duo/chat/react_executor_spec.rb