fix(opencode): revert from the first removed assistant message

[maria-rcks]

opencode rollback used the last retained message and omitted the required id when removing all turns. target the first removed assistant message and read back the native revert boundary, including when multiple assistant messages share a preceding user message. opencode keeps its reverted history until the next prompt.

verified with all 108 opencode adapter tests, server typecheck, and scoped lint and formatting checks. the existing mock follows native inclusive revert semantics, including shared user-message boundaries and consecutive rollbacks.

fixes #2789.

implemented with gpt-6-astra in codex.

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Note

Medium Risk
Changes rollback/read semantics for OpenCode threads and adds a session.get on every readThread; callers that assumed pre-revert thread shape after rollback may see fewer turns.

Overview
Fixes OpenCode thread rollback so it matches native revert semantics instead of slicing messages locally or reverting to the last kept assistant turn.

readThread now loads session metadata and stops listing assistant turns at session.revert.messageID, because OpenCode keeps reverted messages in the transcript until the next prompt.

rollbackThread derives the revert target from the current snapshot: the assistant turn at index length - numTurns (the first turn being removed). It always calls session.revert with that message ID, then re-reads the thread so a boundary that lands on the preceding user message is reflected. When there is nothing to revert, it returns the snapshot without calling revert.

Tests extend the runtime mock to expose revert.messageID on session.get and simulate inclusive revert behavior, with broader coverage for numTurns, consecutive rollbacks, and shared user-message boundaries.

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Note

Fix OpenCodeAdapter rollback to revert from first removed assistant message

• Updates the OpenCode runtime test double to model native revert as a persisted session boundary reported by session.get, instead of deleting messages immediately
• Adds test cases covering zero through three requested turns, repeated rollback/readback, a shared user-message boundary, and an empty thread
• Fixture now includes user messages and assistant parts so the mock can determine the native revert boundary
• Risk: session.revert in the mock now requires a message ID and fails without one; calls without parts no longer truncate the mock message list directly

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[macroscopeapp]

Approvability

Verdict: Approved at 0c7358f

Macroscope's review found this PR approvable — This is a focused OpenCode rollback bug fix that preserves existing APIs while correcting native revert-boundary handling and returning the actual retained turns. The production change is confined to the adapter and is covered by detailed rollback tests.

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Dismissing prior approval to re-evaluate 0c7358f