Resolve "Add feature flag for Fireworks/Codestral 25.01 code completion"

What does this MR do and why?

Depends on AIGW MR gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!2219 (merged)

Adds two feature flags

use_fireworks_codestral_code_completion to enable Fireworks/Codestral code completion per-user code_completion_opt_out_fireworks to opt-out groups/instance from using Fireworks (intended to eventually replace code_completion_model_opt_out_from_fireworks_qwen)

How to set up and validate locally

Request

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"
}'

1. Feature.enable(:use_fireworks_codestral_code_completion)
2. Response should contain model.name: "text-completion-fireworks_ai/codestral-2501"
3. Feature.enable(:code_completion_opt_out_fireworks)
4. Response should not contain model.name: "text-completion-fireworks_ai/codestral-2501"

MR acceptance checklist

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Related to #525571