core: standalone signed-empty reasoning turns are replayed into later requests

Description

V2 replays completed assistant turns whose only content is empty reasoning plus provider metadata.
In packages/core/src/session/runner/to-llm-message.ts, same-model reasoning retains provider metadata, and the meaningful filter considers metadata sufficient even when part.text === "". The existing omits empty assistant turns test explicitly preserves a standalone { text: "", state: { signature } } turn.
The Amazon Bedrock adapter serializes each retained part as:
{
"reasoningContent": {
"reasoningText": {
"text": "",
"signature": "..."
}
}
}
Controlled forks isolated the effect:

• The exact prompt with the clean full history (~620k provider tokens) produced visible text and tools.
• The same prompt with one or two standalone signed-empty turns still worked.
• The same prompt with five accumulated signed-empty turns deterministically returned another signature-only end_turn with no visible output.
• Manual compaction repaired the session because compaction rendering drops empty reasoning parts.
  Expected: signed reasoning state is retained when it accompanies semantic assistant output—text, readable reasoning, or a tool call—but a standalone metadata-only assistant turn is omitted from future model history or marked failed by the no-output handling described separately.
  Actual: state-only turns accumulate and can poison subsequent same-model requests.

Plugins

No response

OpenCode version

opencode2 v0.0.0-dev-18900; current V2 source af332a2363 remains affected.

Steps to reproduce

1. Create a V2 session using a provider that returns signed reasoning metadata.
2. Persist several completed assistant turns containing only reasoning.text = "" and a provider signature.
3. Submit a prompt that works in a fork before those turns.
4. Observe the provider request replay each empty reasoningText plus signature.
5. Observe the later response repeat the signature-only pattern.
6. Compact the same session and retry; the prompt works because the empty reasoning turns are no longer replayed.

Screenshot and/or share link

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Operating System

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Terminal

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