fix(think): preserve regeneration branches after restart

[ben-reitz]

Stacked on #2038.

What users see

The parent PR fixes regenerated answers using the wrong conversation branch. One restart case remained: if the server's Durable Object restarted while a regenerated answer was streaming, the recovered text could be saved beneath the original answer instead of beside it.

flowchart TB
  subgraph after["After"]
    AU["User message"] --> AO["Original answer"]
    AU --> AR["Recovered regeneration"]
  end

  subgraph before["Before"]
    BU["User message"] --> BO["Original answer"]
    BO --> BR["Recovered regeneration"]
  end

Loading

Why it happens

The streamed text survives a restart because it is stored in SQLite. The message it belongs beside did not: that link only lived in memory. After a restart, Think still had the words, but had forgotten where to put them.

The fix

Resumable streams now save that optional parent link with their existing metadata. When Think rebuilds an interrupted answer, it reads the link back and restores the answer to the right branch.

Existing rows keep the old fallback behavior. Conversations that do not branch also keep using the existing schema until they actually need this field.

Tests

A new WebSocket test disables automatic chat recovery, forces a Durable Object restart, acknowledges the resumed stream, and checks that the recovered answer is a sibling of the original. Migration tests cover both old databases and ordinary linear streams.

Suites: think 925 · agents workers 1746 · agents chat 514 · ai-chat workers 655 · pnpm run check.

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[devin-ai-integration]

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