Add alternative response pager integration for retry feature

Summary

Integrate alternative response paging with duo-ui components to allow users to navigate between different LLM response variants from retry attempts.

• Update workflow_utils.js to transform alternatives array on messages
• Add transformAlternatives method to convert backend format
• Track selectedAlternatives state in state manager
• Pass selectedAlternatives prop through view components
• Handle select-alternative event to update displayed variant
• Add unit tests for alternatives transformation (8 new tests)

The selected alternative becomes the canonical response used for follow-up conversation context with the LLM.

Related MRs

• duo-ui rendering: duo-ui!581 (merged)
• Backend ai-assist: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!5801 (closed)
• Backend GitLab: !238143 (closed)

references:

• issue: Browse previous attempts after retry (#593010 - closed)
• epic: Manual Retry for Duo Chat Responses (gitlab-org#21289 - closed)

Test plan

• Unit tests pass for transformAlternatives method
• Unit tests pass for transformChatMessages with alternatives
• Integration testing with duo-ui component
• E2E testing with backend alternatives support

MR acceptance checklist

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Changelog: added