core: signature-only responses complete successfully with no visible output

Description

V2 treats a provider response containing only empty reasoning metadata as successful user-visible output.
packages/core/src/session/runner/publish-llm-event.ts sets outputStarted = true on reasoning-start, even if no non-empty reasoning delta arrives. If the provider subsequently emits a clean step-finish (stop / raw end_turn), packages/core/src/session/runner/llm.ts records Step.Ended successfully without checking for text, readable reasoning, or a tool call.
The resulting assistant message can contain only:
{
"finish": "stop",
"rawFinish": "end_turn",
"content": [
{ "type": "reasoning", "text": "", "state": { "signature": "..." } }
]
}
The TUI renders no answer and no error, so the operation appears to have silently done nothing.
Expected: a clean finish with no non-empty text/reasoning and no tool call is surfaced as a no-output provider failure or otherwise handled explicitly. A metadata-only reasoning event should not count as visible output.
Actual: the step succeeds and the session becomes idle with no renderable response.

Plugins

No response

OpenCode version

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

Steps to reproduce

1. In a runner test, stream step-start.
2. Stream reasoning-start with provider metadata containing a signature.
3. Stream an empty reasoning delta and reasoning-end.
4. Stream step-finish with normalized stop and raw end_turn.
5. Read the projected assistant message and session outcome.
6. Observe a successful assistant step with no visible content or error.

Screenshot and/or share link

No response

Operating System

Ubuntu 24.04.4 LTS, Linux 5.15.0-91-generic, x86_64

Terminal

TERM=xterm-256color; shell /usr/bin/fish