Replace hardcoded model placeholder with 'agentic-model' slot to allow
parent components to provide custom model selection dropdown UI.

- Add agentic-model slot in web_agentic_duo_chat.vue
- Update stories to demonstrate new slot usage

chore(deps): update dependency @gitlab/svgs to ^3.149.0

See merge request !319

chore(deps): update dependency @gitlab/svgs to ^3.149.0

See merge request !320

chore(deps): update dependency @gitlab/ui to ^123.10.1

See merge request !321

chore(deps): update playwright to v1.55.1

See merge request !289

chore(deps): update dependency node to v22.20.0

See merge request !292

chore(deps): update dependency @gitlab/ui to ^123.11.0

See merge request !322

chore(deps): update playwright to v1.56.1

See merge request !323

feat(AgenticDuoChat): add model selection slot

See merge request !318

# [12.4.0](v12.3.3...v12.4.0) (2025-10-21)

### Features

* **AgenticDuoChat:** add model selection slot ([84914385](84914385))