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

[github-actions]

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

Based on my search, I found potential related PRs:

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

   • Very closely related; this appears to be a prior attempt to address empty provider output failures.

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

   • Related to handling incomplete provider responses that should be retried rather than ending the session.

3. fix(opencode): drop truncated reasoning from replayed history #40148 - fix(opencode): drop truncated reasoning from replayed history

   • Related to handling empty or incomplete reasoning-only responses.

4. fix(core): recover from truncated tool arguments with bounded retries #37220 - fix(core): recover from truncated tool arguments with bounded retries

   • Related pattern of using bounded retries for provider output issues.

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

   • Older related fix for handling empty stream truncations with retry logic.

The most notable is #37843 which directly addresses failing on empty provider output — this PR (40511) may be building upon or superseding that approach with the addition of bounded retry behavior.