fix(opencode): retry empty unknown responses

[3351163616]

Issue for this PR

Closes #41469

Type of change

• Bug fix

What does this PR do?

When a provider returns an empty completion (0 tokens, no text/reasoning/tool output, finish reason maps to unknown), opencode records it as a normal completed turn and the session loop exits silently — no error surfaced, no retry. The conversation appears to stop mid-task.

This PR makes the processor treat such turns as transient failures:

• packages/opencode/src/session/processor.ts: track whether the stream generated any text/reasoning/tool output; if finish === "unknown" and nothing was generated, throw SessionRetry.EmptyResponseError.
• packages/opencode/src/session/retry.ts: add EmptyResponseError and retry it like other transient failures (existing backoff policy).
• Test added covering the empty-response retry path.

This is a rebased port of #40531 (same fix, currently conflicting with dev) so it is mergeable as-is.

How did you verify your code works?

• New test passes: bun test session/processor-effect.test.ts -t "retry empty"
• Full suite: bun test session/processor-effect.test.ts — 16 pass / 1 fail (the failure is the pre-existing midstream server errors test, failing on dev without this change too)
• bun typecheck passes

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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

Based on my search, here are the related PRs:

Related/Previous Work:

• fix(opencode): retry empty unknown responses #40531 — fix(opencode): retry empty unknown responses — This is the original PR that fix(opencode): retry empty unknown responses #41466 is rebasing. The description states fix(opencode): retry empty unknown responses #40531 is conflicting with dev, so fix(opencode): retry empty unknown responses #41466 is a rebase of the same fix on the current dev state.

• fix(opencode): make long-lived provider streams robust to silent SSE terminations #39970 — fix(opencode): make long-lived provider streams robust to silent SSE terminations — Related to handling provider stream failures and silent terminations.

• fix(opencode): surface truncated turns instead of ending the loop #40142 — fix(opencode): surface truncated turns instead of ending the loop — Related handling of incomplete/truncated responses.

• fix(opencode): surface empty model responses #37732 — fix(opencode): surface empty model responses — Earlier work on surfacing empty model responses.

Note: PR #41466 (the current PR) appears in the results because the search returns it, but this is expected since it's the current PR being checked.

The key relationship is with #40531, which is the original implementation that this PR rebases. As noted in the PR description, #41466 applies the same fix to the current dev state and should be mergeable.

[github-actions]

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