fix(think): preserve regeneration branch context

[ben-reitz]

Stacked on #2120.

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 history also remains the inference source when lifecycle hooks or context compaction prepare the turn.

Testing

• 87 client-tools tests
• model-boundary regression verifies the prior assistant is absent from regenerated inference context
• Session assertions verify the previous and regenerated assistants remain siblings

Fixes #2028

Stack

1. fix(agents): keep session compactions branch-local #2120 — Session branch-local compaction
2. This PR — Think regeneration context
3. fix(think): preserve active transcript during repair #2121 — transcript repair ownership
4. fix(think): preserve branches during recovery #2122 — branch-aware recovery
5. fix(think): preserve regeneration branches after restart #2057 — restart durability

[changeset-bot]

🦋 Changeset detected

Latest commit: fcda26e

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

This PR includes changesets to release 3 packages

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

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