test(executors): restore the #10986 reasoning-only fallback guards (#12364)

The #10265 rewrite of command-code-executor.test.ts (b6412c6fe) deleted the two regression tests #10986 added for reasoning-only Command Code output, while the production fallback in createJsonResponse / createStreamResponse survived — leaving it unguarded. Both are restored, now routed through the /alpha/generate fallback that is the only way to reach the CLI translator since #10265, via a shared goPlanFallbackFetch() helper.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.