Store additional context into chat history

What does this MR do and why?

Store additional context into Duo Chat history

Related to #480702 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

REST endpoint

1. Run bundle exec rails c
2. Call chat completion:

   user = User.find_by_username('<YOUR-USERNAME>')
   safe_params = { content: 'please explain the following code', additional_context: [{ category: 'snippet', id: 'print', content: 'puts "Hello world"', metadata: "{\"freeform\":\"json\"}" }] }
   ::Gitlab::Duo::Chat::Completions.new(user).execute(safe_params: safe_params)

3. Read stored messages

   Gitlab::Llm::ChatStorage.new(user).messages

You should receive recent messages for the user, one of them should contain additional_context. For example:

 #<Gitlab::Llm::ChatMessage:0x000000017ccfece8
  @additional_context=[{"id"=>"print", "category"=>"snippet", "content"=>"puts \"Hello world\"", "metadata"=>"{\"freeform\":\"json\"}"}],
  @agent_version_id=nil,
  @ai_action="chat",
  @content="please explain the following code",
  @errors=[],
  @extras={"has_feedback"=>false},
  @id="fae26fd5-6ca2-427f-b995-1273854f2428",
  @request_id="df88b042-dc46-4822-82db-b86bdfce98d7",
  @role="user",
  @timestamp=Thu, 05 Sep 2024 08:52:51.000000000 UTC +00:00,
  @user=#<User id:45 @ck3g>>,

GraphQL mutation

1. Run bundle exec rails c
2. Find a required user user = User.find_by_username('<YOUR-USERNAME>')
3. Make sure feature flag is enabled Feature.enable(:duo_additional_context, user)
4. In your browser make sure you're logged in at http://gdk.test:3000 as a user from previous steps
5. Navigate to http://gdk.test:3000/-/graphql-explorer
6. Run the following mutation

   mutation chatAction {
     aiAction(input: {chat: {content: "Explain this code", additionalContext: [
       {
         category: SNIPPET, id: "hello world", content: "puts 'Hello, world'",
         metadata: "{\"freeform\":\"json\"}"
       }
     ]}}) {
       clientMutationId
       errors
     }
   }

7. Back in Rails console, call the following method Gitlab::Llm::ChatStorage.new(user).messages

You should receive recent messages for the user, one of them should contain additional_context. For example:

 #<Gitlab::Llm::ChatMessage:0x000000017ca32aa0
  @additional_context=[{"id"=>"hello world", "category"=>"snippet", "content"=>"puts 'Hello, world'", "metadata"=>"{\"freeform\":\"json\"}"}],
  @agent_version_id=nil,
  @ai_action="chat",
  @content="Explain this code",
  @errors=[],
  @extras={"has_feedback"=>false},
  @id="7cc9ae62-d69a-4e41-80b5-ce1922df8798",
  @referer_url="http://gdk.test:3000/-/graphql-explorer",
  @request_id="c60ec3e1-39e0-407a-92da-cb75dcbbed7a",
  @role="user",
  @timestamp=Thu, 05 Sep 2024 08:49:23.000000000 UTC +00:00,
  @user=#<User id:45 @ck3g>>,