V2: port transform and interruption durability across both model paths

[opencode-agent]

Scope

Port the existing transform and interruption durability behavior to V2 in packages/core. This must cover both model execution paths:

• the @opencode-ai/llm path;
• the AI SDK path.

Neither path should have weaker persistence or recovery semantics.

Requirements

• Persist transform results needed for deterministic replay/recovery before dependent continuation or side effects.
• Represent interruption durably and settle any in-flight assistant/tool state without leaving orphaned or permanently pending parts.
• After replay or restart, project the same terminal/interrupted state rather than a successful-looking incomplete message.
• Keep the durability contract aligned across both paths, ideally behind shared core behavior rather than two drifting implementations.
• Add a parity test matrix covering normal completion, user interruption, and interruption followed by replay/restart for each path.

Related lifecycle/error-event design: #35016
Related typed LLM error work: #34995

Source: https://discord.com/channels/1391832426048651334/1447692703549755466/1523404602102517941