fix(ai): recover missing reasoning item ids

[rekram1-node]

Summary

• track Responses reasoning items by their wire output index when a provider omits the item ID
• reuse deterministic local block identities across summary parts, deltas, finals, and item completion
• retain provider-issued IDs separately when they arrive later, including subsequent events without an output index
• reject reasoning item boundaries that provide neither an item ID nor an output index

Scope

This PR only repairs reasoning stream identity. Final-item text reconciliation and terminal encrypted-content backfill remain separate follow-ups.

Verification

• bun run typecheck
• OpenAI Responses tests
• generic Open Responses compatibility tests
• xAI Responses tests

All 123 focused tests pass.