Add SSE response summary helpers

[baofeidyz]

Summary

• Add SSE response summary controls and rendering for event streams
• Move reusable SSE summary parsing helpers into @yaakapp-internal/sse
• Support built-in provider paths and custom result key paths without changing raw/event inspection behavior

Attribution

• All changes in this pull request were produced by Codex using GPT-5.5.

Verification

• npm run -w @yaakapp/yaak-client lint
• npm exec -- vp check --fix apps/yaak-client/components/responseViewers/EventStreamViewer.tsx apps/yaak-client/hooks/useSseSummaryResultKeyPath.ts
• Built local macOS app bundle with npm run client:build -- --bundles app

[baofeidyz]

@gschier Hello, I'd like to ask if it's possible to merge this merge request into the main branch? If not, could you please let me know why or how I should adjust it? Thank you very much.