feat(chat): add alternative response pager for retry feature

Summary

Add AlternativePager component to navigate between alternative LLM responses from retry attempts.

• Add AlternativePager component with chevron navigation (< 1/2 >)
• Add prepend slot to MessageActionBar for pager placement (left-most position)
• Update DuoChatMessage to support alternatives prop and selected index tracking
• Update DuoChatConversation to pass selectedAlternatives state and bubble up selection events
• Add Storybook stories and unit tests (17 tests for AlternativePager)

The pager allows users to navigate through different response variants when using the retry feature. Whichever alternative is displayed becomes the selected response for follow-up conversation context.

references:

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

Screenshots/Demo

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Test plan

• Unit tests pass for AlternativePager component
• Unit tests pass for MessageActionBar prepend slot
• Verify Storybook stories render correctly
• Integration testing with gitlab repo MR

Related

• Integration MR: TBD (gitlab repo)