Add "disable_code_gecko_default" feature flag

What does this MR do and why?

Adds the ability to disable code gecko as the default code completion model using a feature flag disable_code_gecko_default

In non-asia regions, it will default to vertex_ai/codestral-2501, in asian regions, it will default to fireworks_ai/qwen2p5-coder-7b

This changes the default, so any other feature flags relating to code completion (i.e. fireworks_qwen_code_completion or amazon_q_chat_and_code_suggestions) need to be disabled for this to work

Depends on AIGW MR gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1993 (merged)

MR acceptance checklist

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How to set up and validate locally

Code Suggestions API

curl -X "POST" "http://gdk.test:3000/api/v4/code_suggestions/completions" \
     -H 'Content-Type: application/json' \
     -H 'Authorization: Bearer AUTH_TOKEN' \
     -d $'{
  "current_file": {
    "file_name": "test.go",
    "content_above_cursor": "// write a function that parses a .env file",
    "content_below_cursor": "//end function"
  },
  "prompt_version": 2,
  "intent": "completion"
}'

should return "model.name": "vertex_ai/codestral-2501",

Direct Access

curl -X "POST" "http://gdk.test:3000/api/v4/code_suggestions/direct_access" \
     -H 'Content-Type: application/json' \
     -H 'Authorization: Bearer AUTH_TOKEN' \
     -d $'{}'

Should return "headers.x-gitlab-enabled-feature-flags": "disable_code_gecko_default"

Related to #519375 (closed)