fix(web): group streamed run message deltas

[kapelame]

Summary

• Group message.delta events into one raw event block per run step.
• Concatenate streamed text in event order while retaining the latest payload metadata.
• Keep non-delta raw event blocks, styling, and interactions unchanged across harnesses.
• Keep the formatter outside the oversized RunsPage.tsx.

Scope

In scope: admin run raw-event formatting and rendering only.

Out of scope: backend or API changes, state management, harness-specific branches, and UI restyling.

Testing

• Targeted formatter assertions for grouped deltas and unchanged non-delta events
• pnpm --filter @parsar/web typecheck
• pnpm --filter @parsar/web build
• pnpm --filter @parsar/web exec eslint src/lib/agent-run-event-format.ts src/pages/admin/RunsPage.tsx --rule 'react-hooks/set-state-in-effect: off'
• make check (passed; Docker was unavailable, so the repository script skipped its Postgres migration smoke test)

Review

Two independent blind reviews of the complete diff found no issues.