fix(runtime): persist final assistant log snapshots

[Yevanchen]

Summary

• persist the canonical final assistant snapshot with run completion
• reuse the existing stream fold to collapse fractured deltas into one log entry
• avoid duplicating snapshots already supplied by a runtime

Root cause

OpenAI runtime completed with a canonical session message but no authoritative message.added event, so the Logs view could only render fractured deltas.

Verification

• just test-file apps/api/tests/runtime-final-output-ingestion.test.ts
• just tc-package @mosoo/api
• just check reached 1080 passing tests; one unrelated macOS /var versus /private/var path assertion failed in apps/driver/tests/acp-file-system.test.ts

No migration, historical rewrite, or deployment. Incident record: #460.