feat(chat): add model metadata to chat endpoint

What does this merge request do and why?

Related issue: Add self-hosted models support for v2/agents (#513 - closed)

This MR adds support for custom models to v2/chat/agent endpoint: it's model_metadata field added to the options.

Example:

{
  "prompt": "string",
  "options": {
    "chat_history": "string",
    "agent_scratchpad": {
      "agent_type": "react",
      "steps": [
        {
          "thought": "string",
          "tool": "string",
          "tool_input": "string",
          "observation": "string"
        }
      ]
    },
    "context": {
      "type": "issue",
      "content": "string"
    },
    "model_metadata": {
      "endpoint": "http://127.0.0.1:11434/v1",
      "name": "mistral",
      "provider": "openai",
      "api_key": "string"
    }
}

Testing

Please add the following diff on top of this MR: Execute chat requests via new endpoint in AI Ga... (gitlab-org/gitlab!150529 - merged)

Diff

diff --git a/ee/lib/gitlab/llm/chain/requests/ai_gateway.rb b/ee/lib/gitlab/llm/chain/requests/ai_gateway.rb
index 3035087ffc2d..7c9065dbfdbe 100644
--- a/ee/lib/gitlab/llm/chain/requests/ai_gateway.rb
+++ b/ee/lib/gitlab/llm/chain/requests/ai_gateway.rb
@@ -127,6 +127,19 @@ def model_params(options)
             end
           end
 
+          def model_metadata_params(options)
+            return unless chat_feature_setting&.self_hosted?
+
+            self_hosted_model = chat_feature_setting.self_hosted_model
+
+            {
+              provider: :openai, # for self-hosted models we support Messages API format at the moment
+              name: self_hosted_model.model,
+              endpoint: self_hosted_model.endpoint,
+              api_key: self_hosted_model.api_token
+            }
+          end
+
           def request_body_chat_2(prompt:, options: {})
             {
               prompt: prompt,
@@ -139,7 +152,8 @@ def request_body_chat_2(prompt:, options: {})
                 context: {
                   type: options[:current_resource_type],
                   content: options[:current_resource_content]
-                }
+                },
+                model_metadata: model_metadata_params(options)
               }
             }
           end