fix(command-code): surface reasoning-only output as content when no text-delta (#10986)

[diegosouzapw]

Closes #10986

Root cause

Command Code's /alpha/generate executor (open-sse/executors/commandCode.ts) aggregates only text-delta events into message.content. When a model (e.g. meta/muse-spark-1.2-contributor) emits its whole answer as reasoning-delta events and never a text-delta, state.content stays "" and the client gets a useless/empty content field with HTTP 200 — even though a real answer exists in reasoning_content. The report's metrics (Total Out: 64 / Reasoning: 61) match exactly.

Both createJsonResponse and createStreamResponse had the gap. The combo-routing validator (open-sse/services/combo/validateQuality.ts, from #2341/#3587) already defends this shape for combo-routed requests, but a direct single-model request bypasses it entirely.

Fix

Scoped to the executor (surgical, HR#18): when state.content is empty AND state.reasoning is non-empty AND there are no tool calls (the exact detection #3587 uses), fall back message.content to the reasoning text. reasoning_content stays populated for reasoning-aware clients. Behavior for the common text-delta case is unchanged (existing test text-delta + reasoning-delta mixed passes unmodified).

• createJsonResponse: content fallback on the assembled message.
• createStreamResponse: track whether any content delta was emitted; emit one synthetic content delta from the reasoning buffer at finish (and at stream-end) when none was sent.

Regression test (TDD, proved RED then GREEN)

tests/unit/command-code-executor.test.ts — 2 new cases (non-stream JSON + SSE-stream), added after a standalone probe confirmed RED (expected message.content to carry usable text, got: ""). Full file: 22/22 pass.

Gates

• file-size: commandCode.ts rebaselined 1038→1059 with _rebaseline_2026_08_21_10986_reasoning_only_content justification (mirrors fix(combo): clear stale sticky pins when stickiness is disabled #10907/feat(vision-bridge): honor configured vision model + support custom model input & all active providers (#10808, #10809) #10859 pattern).
• complexity (cyclic) OK — 2620 < 2774 baseline.
• cognitive-complexity OK — 1180 < 1223 baseline.
• typecheck:core clean; eslint clean on changed files.
• changelog-integrity OK (fragment changelog.d/fixes/10986-reasoning-only-content.md).
• sibling executor tests: command-code-vision/usage/auth-assist/executor-command-code/registry-vision/issue-6623 → 81/81 pass.

⚠️ base-red inherited: #9985 — src/lib/modelCapabilities.ts file-size drift (1072 > 1016 frozen) is pre-existing on the base tip (git show origin/release/v3.8.50 = 1071), not touched by this diff.