fix(task): report error to parent when subagent finishes with error (#52378) - #52678

The following comment was made by an LLM, it may be inaccurate:

The explicit finish: "error" check addresses the old task-tool path, but this PR targets dev/packages/opencode. The current contribution guide asks for v2, where subagent jobs run through packages/core instead.

On current V2 55dde881, the successfully ended stream synthesizes an assistant failure only for finish === "unknown"; "error" still does not enter that branch. The V2 reproduction already shared in #52378 exercises the real runner, job and database for both foreground and background tasks. The stubbed TaskPromptOps test here does not cover that path. The issue also has a separately reproduced length case; it needs an explicit scope decision rather than being treated as covered by the error-only check.

Would you be able to port the fix to the V2 owner and reuse packages/core/test/tool-subagent.test.ts for both delivery modes? This comment is based on the current source comparison and the linked earlier synthetic event reproduction, not a new live-provider test. AI-assisted review; no competing PR opened.

@r266-tech Ported to the V2 owner path as requested, pushed to this branch:

• packages/core/src/session/runner/llm.ts now synthesizes an assistant failure (InvalidProviderOutputReason) when a successfully ended stream records finish unknown or error, and suppresses Step.Ended/continuation after that failure so it surfaces as a terminal step instead of silent success.
• packages/core/src/session/runner/publish-llm-event.ts exposes hasAssistantFailure used for the suppression.
• Coverage: new SessionRunnerLLM case projects an error finish as a terminal assistant step failure (foreground path), plus a background delivery regression in packages/opencode/test/tool/task.test.ts asserting the parent job ends in error state with the task_id message. The original foreground task test is untouched.
• Scope decision on length: explicitly left out, with a code comment — length is a valid truncation outcome that may carry usable partial output, so it stays outside this subagent-failure fix.

Verification on this box: packages/opencode task suite 24/24 green, packages/core session-runner suite 88/88 green, scoped tsgo typecheck clean for both packages. Repo-wide bun typecheck cannot complete in this environment (tsgo exceeds 300s and gets SIGTERM), so that one is left to CI.

This branch has been upgraded to V2. We're no longer taking PRs for V1. If you think this is still relevant, please port it over and open it for V2.

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