Remove the use_vertex_codestral_code_completion ops flag

What does this MR do and why?

This MR removes the ops flag use_vertex_codestral_code_completion and its code paths. !251313 (merged) introduced the flag. It defaulted to off, and no environment enabled it.

The flag let the monolith serve Vertex-hosted Codestral 25.08 (gitlab / codestral_2508_vertex) instead of Fireworks-hosted Codestral 25.08 (fireworks_ai / codestral-2508). This applied to the /code_suggestions/direct_access payload and the v2 completions proxy body. Google caps Codestral on Vertex at 2.2M input tokens per minute per region, so a full default flip is not possible. The Fireworks/Vertex split will instead use a weighted default_models list in the AI gateway. A per-user monolith flag would bypass those gateway weights, so it has no future use.

This MR deletes config/feature_flags/ops/use_vertex_codestral_code_completion.yml. It removes the flag branches from current_model and saas_primary_model_class in ee/lib/code_suggestions/model_details/code_completion.rb, plus the two helper methods behind them. It removes the flag from the GDK exclusion list in ee/lib/gitlab/duo/developments/feature_flag_enabler.rb. It removes the flag contexts from three spec files. Behavior does not change: the flag was off everywhere, so default code completion already used fireworks_ai / codestral-2508. The flag never affected pinned models, self-hosted models, or Amazon Q. The prompt class CodeSuggestions::Prompts::CodeCompletion::VertexCodestral predates the flag and stays.

After merge, run /chatops gitlab run feature delete use_vertex_codestral_code_completion --dev --pre --staging --staging-ref --production. This MR closes the rollout issue.

References

• Closes #619299 (closed)
• https://gitlab.com/gitlab-org/gitlab/-/issues/614058
• !251313 (merged)
• https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/work_items/2714
• https://gitlab.com/groups/gitlab-org/modelops/ai-model-validation-and-research/ai-evaluation/-/work_items/60

Screenshots or screen recordings

Not applicable. This is a backend-only change with no UI change.

How to set up and validate locally

1. Run git grep use_vertex_codestral. It returns no results.
2. Run bundle exec rspec ee/spec/lib/code_suggestions/model_details/code_completion_spec.rb ee/spec/lib/code_suggestions/tasks/code_completion_spec.rb ee/spec/requests/api/code_suggestions_spec.rb. It passes 273 examples with 0 failures.
3. Send POST /api/v4/code_suggestions/direct_access. It still returns model_details with model_provider: fireworks_ai and model_name: codestral-2508.

MR acceptance checklist

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

RuboCop is clean on the five changed Ruby files. The three spec files pass locally.