Make model selection feature flags for .com and SM default enabled and GA ready
<!-- Title suggestion: [Feature flag] Cleanup <feature-flag-name> -->
## Summary
This issue is to cleanup the `duo_agent_platform_model_selection` and `ai_user_model_switching` feature flag, after the feature flags have been enabled by default for an appropriate amount of time in production.
- `duo_agent_platform_model_selection` gates [DAP namespace model selection for .com](https://gitlab.com/gitlab-org/gitlab/-/issues/568112)
- `ai_user_model_switching` gates [end-user model selection in .com](https://gitlab.com/groups/gitlab-org/-/epics/19251)
Both have been enabled globally via chatops on 18/09/2025. See [roll out issue](https://gitlab.com/gitlab-org/gitlab/-/issues/568855) for more details.
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## Owners
- Team: Custom Models
- Most appropriate slack channel to reach out to: `#g_custom_models`
- Best individual to reach out to: @julie_huang / @manojmj
- PM: @susie.bee
## Stakeholders
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Are there any other stages or teams involved that need to be kept in the loop?
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- The Support Team
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## Expectations
### What might happen if this goes wrong?
<!-- Any MRs that need to be rolled back? Communication that needs to happen? What are some things you can think of that could go wrong - data loss or broken pages? -->
### Cleaning up the feature flag
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- [ ] Specify in the issue description if this feature will be removed completely or will be productized as part of the Feature Flag cleanup
- [ ] Create a merge request to remove `<feature-flag-name>` feature flag. Ask for review and merge it.
- [ ] Remove all references to the feature flag from the codebase.
- [ ] Remove the YAML definitions for the feature from the repository.
- [ ] Create [a changelog entry](https://docs.gitlab.com/ee/development/feature_flags/#changelog).
- [ ] Ensure that the cleanup MR has been deployed to both production and canary.
If the merge request was deployed before [the code cutoff](https://about.gitlab.com/handbook/engineering/releases/#self-managed-releases-1),
the feature can be officially announced in a release blog post.
- [ ] `/chatops run auto_deploy status <merge-commit-of-cleanup-mr>`
- [ ] Close [the feature issue](ISSUE LINK) to indicate the feature will be released in the current milestone.
- [ ] If not already done, clean up the feature flag from all environments by running these chatops command in `#production` channel: `/chatops run feature delete <feature-flag-name> --dev --pre --staging --staging-ref --production`
- [ ] Close this rollout issue.
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/label ~"type::feature" ~"feature::addition"
/label ~"type::maintenance"
/label ~"type::bug"
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issue