fix(core): fail steps with empty provider output after bounded retries

[ServOMorph]

Issue for this PR

Closes #37372

Type of change

• Bug fix

What does this PR do?

A provider turn that streams a successful step finish but produces no visible text and no tool call (for example a reasoning-only response) was recorded as a successful step (finish: "stop"), so the session ended cleanly with an empty assistant turn.

The publisher now tracks whether any usable output was streamed: non-blank text deltas or a tool call. When a successful stream has neither, the turn is retried up to two times through the existing bounded turn-transition path. If the budget is exhausted, the step is recorded as a terminal failure (finish: "error") with a clear message instead of a false success.

How did you verify your code works?

• Ran the session-runner tests in packages/core (86 tests in session-runner.test.ts, 129 across the runner files) — all green.
• bun typecheck in packages/core — clean.
• Updated existing empty-turn fixtures to produce real output and added tests for retry-then-fail, retry-then-recover, and tool-call (no retry).

Screenshots / recordings

N/A — core change, no UI.

Checklist

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

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Related PRs Found

These PRs address similar issues with empty or incomplete provider output:

1. PR fix(opencode): surface truncated turns instead of ending the loop #40142 - fix(opencode): surface truncated turns instead of ending the loop

   • Related: Both handle cases where provider output is incomplete or missing

2. PR 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: Handles edge cases in provider stream handling

3. PR fix(core): fail empty provider output #37843 - fix(core): fail empty provider output

   • Related: Directly addresses failing on empty provider output (may be a precursor or related approach)

4. PR fix(session): retry empty stream truncations and discard partial parts #26167 - fix(session): retry empty stream truncations and discard partial parts

   • Related: Handles retrying when streams produce incomplete output

The most relevant duplicate candidate is PR #37843 which also tackles failing on empty provider output. This PR (40437) appears to be a more comprehensive fix with bounded retries and better tracking of usable output.

[github-actions]

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