fix(core): fail length finishes that return no content

[Veld101]

Issue for this PR

Closes #50949

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

A provider can finish a Step normally with finish_reason: "length" while streaming no text, no reasoning, and no tool call, and with zero output tokens. The publisher settled that Step as a success, so the turn ended with no message, no error, and a successful exit — indistinguishable from a valid empty answer, and impossible to diagnose from the TUI/CLI.

Mirroring the existing content-filter handling, a normalized length finish with no streamed output now fails the Step with a visible error:

The model returned no content (finish reason: length)

Partial output is unaffected: a length finish that streamed any content keeps its normal settlement and visible text, and a length finish with tool calls still continues. specs/v2/session.md is updated alongside.

The error uses a new provider.incomplete type. SessionError.Error.type is a free-form string, so no schema change or client regeneration is required; the failure surfaces through the existing Step failure path with the finish reason in its message.

How did you verify your code works?

• bun test test/session-runner-tool-events.test.ts — 28 pass (3 new cases: empty length fails the step; length with streamed text settles normally; length with a tool call keeps continuation)
• bun test test/session-runner.test.ts test/session-step.test.ts test/session-runner-tool-events.test.ts — 252 pass / 0 fail (1 new end-to-end scenario asserting a visible terminal failure and no session.step.ended)
• bun typecheck in packages/core, and bun turbo typecheck --filter=@opencode/core
• bun run lint — 0 warnings / 0 errors

Note: bun run check could not complete in my environment because packages/app/src/custom-elements.d.ts is a git symlink (mode 120000) that my Windows checkout materialized as a plain text file (core.symlinks=false), so @opencode/app#typecheck fails with TS1128. That is unrelated to this change and passes on a normal checkout; @opencode/core typecheck passes.

Screenshots / recordings

Not applicable; this is not a UI change.

Checklist

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

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Thanks for your contribution!

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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]

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