Set claude 2.1 as model for slash commands

What does this MR do and why?

• The ensure that all tools have a model set
• Otherwise, they use the model default, which is Claude 3 sonnet if the user has a feature flag enabled. But these tools are still sending json request bodies for the completions API.
• See code here for default model config
• We eventually want to migrate these tools to Claude 3, but we want to support them staying on Claude 2.1 for now so we can selectively enroll users into Claude 3 without degrading behavior for existing customers.
• Resolves #454849 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:ai_claude_3_sonnet)

2. Test the slash commands: /explain, /refactor, and /tests. All should work and be using Claude 2.1

Also good to ensure that all anthropic prompts have the model option set:

 git ls-files | grep anthropic.rb
ee/lib/api/ai/experimentation/anthropic.rb
ee/lib/code_suggestions/prompts/code_generation/anthropic.rb

ee/lib/gitlab/llm/chain/agents/zero_shot/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/requests/anthropic.rb
ee/lib/gitlab/llm/chain/tools/ci_editor_assistant/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/epic_identifier/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/epic_reader/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/explain_code/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/issue_identifier/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/issue_reader/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/refactor_code/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/summarize_comments/prompts/anthropic.rb
ee/lib/gitlab/llm/chain/tools/write_tests/prompts/anthropic.rb

We can ignore the first 2 serach results because code suggestions doesn't use the AiGateway::Client and sets model headers here

Manual check for MODEL being set in all other tools:

• ee/lib/gitlab/llm/chain/agents/zero_shot/prompts/anthropic.rb (this prompt works for claude 3 when feature flag enabled so no MODEL needed)
• ee/lib/gitlab/llm/chain/requests/anthropic.rb (not a tool so not applicable)
• ee/lib/gitlab/llm/chain/tools/ci_editor_assistant/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/epic_identifier/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/epic_reader/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/explain_code/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/issue_identifier/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/issue_reader/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/refactor_code/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/summarize_comments/prompts/anthropic.rb
• ee/lib/gitlab/llm/chain/tools/write_tests/prompts/anthropic.rb

ee/lib/gitlab/llm/chain/tools/explain_code/prompts/anthropic.rb

@@ -9,13 +9,15 @@ module Prompts
             class Anthropic
               include Concerns::AnthropicPrompt
 
+              MODEL = 'claude-2.1'
+
               def self.prompt(variables)
                 base_prompt = Utils::Prompt.no_role_text(
                   ::Gitlab::Llm::Chain::Tools::ExplainCode::Executor::PROMPT_TEMPLATE, variables
                 )
                 {
                   prompt: "\n\nHuman: #{base_prompt}\n\nAssistant:",
-                  options: {}
+                  options: { model: MODEL }
                 }
               end
             end