fix(think): preserve regeneration branch context

[ben-reitz]

Bug

Regeneration means “answer this user message again” while keeping the previous answer available. Think already stored the new answer as a sibling, but it generated that answer from the wrong history.

Stored conversation:

Earlier history → User message
                  └─ Old answer  ← active leaf

Expected model input: Earlier history → User message
Actual model input:   Earlier history → User message → Old answer → “Continue…”

The result looked like a new branch in storage, but the model had been asked to continue the old answer instead of producing an independent alternative.

Fix

Think now uses one server-validated branch point for both model input and output storage. For regeneration, model history ends at the selected user message, and the generated answer is stored beneath that same message.

Earlier history → User message
                  ├─ Old answer  ← preserved
                  └─ New answer  ← generated without seeing the old answer

The selected branch is also preserved through hooks, retries, context compaction, Durable Object eviction recovery, and partial-response reconstruction. Re-delivered recovery work is idempotent, so it cannot create duplicate alternatives.

Summary

• generate regenerated responses from the server-selected branch point instead of the previous assistant leaf
• preserve the selected branch through lifecycle hooks, retries, context-overflow compaction, and chat recovery
• persist recovered responses as sibling branches and make branch-scoped recovery retries idempotent
• add model-boundary, Session, and forced-eviction regression coverage

Fixes #2028

Testing

• pnpm run check
• pnpm --dir packages/agents run test:chat — 514 passed
• pnpm --dir packages/think run test:workers — 923 passed

[changeset-bot]

🦋 Changeset detected

Latest commit: 24f92d0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
agents	Patch
@cloudflare/think	Patch
@cloudflare/agent-think	Patch

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commit: 24f92d0

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