Fix streaming UI message dedupe

[robelest]

Summary

• Deduplicate persisted and streaming UI messages before recombining split assistant messages.
• Add regression coverage for a streamed assistant step that duplicates a persisted step.

Fixes #276.

Root Cause

combineUIMessages merges assistant steps that share an order. Running dedupe after that merge loses the original per-step stepOrder, so a streaming duplicate can fail to match the persisted step it replaces.

Validation

• git diff --check
• npm run build
• npm run test
• npm run typecheck
• npm run lint

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@convex-dev/agent@281

commit: fa1c44f

[ianmacartney]

looks reasonable