Restore retry alternatives pager for duoWorkflowBranches API

What does this MR do and why?

The alternatives pager (browse previous manual-retry attempts) was wired end-to-end in !240895 (merged) against a msg.alternatives field a backend MR was going to add directly on each DuoMessage. That backend MR (!238143 (closed)) was closed, and the frontend transform for it was later removed because that GraphQL shape never shipped. Since then nothing populated .alternatives, so the pager never rendered even though duo-ui's components and this app's selectedAlternatives/retryStates/ select-alternative wiring were still present.

The backend has since replaced it with DuoMessage.alternativeCount plus a separate duoWorkflowBranches(workflowId, threadTs) query. This MR adapts that new contract into the shape duo-ui's pager already expects, without changing duo-ui itself:

• Select alternativeCount alongside threadTs/parentTs, and add a query for duoWorkflowBranches.
• Add alternatives_utils.js: collapses same-question retry turns (a live retry appends a new turn rather than replacing the old one in place) into one, attaching an alternatives array to the remaining terminator message. Historical (pre-session) alternatives are fetched lazily, only once a user pages to them.
• Wire the merge into both duo_agentic_chat_state_manager.vue and its light variant.

Only observable once the dw_read_blobs_graphql feature flag (default off) is enabled, so there is no changelog entry.

References

• #593010 (closed)
• gitlab-org#21289 (closed)

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