Fix realtime OpenClaw final text extraction

[rogerchappel]

Summary

• Extract realtime consult final text from OpenClaw content arrays
• Fall back from empty message envelopes to the full final event
• Cover trace artifact assistantTexts payloads from OpenClaw trajectories

Verification

• pnpm test -- 'src/app/api/runtimes/[id]/talk/realtime/relay/route.test.ts'\n- pnpm typecheck\n- git diff --check\n- pre-push hook: pnpm typecheck && pnpm build\n\n## Risk\nLow. This only broadens final text extraction for existing OpenClaw realtime events and preserves the empty-result fallback.