test(opencode): cover ensureThread identity adoption

The adoption change in ensureThread shipped without a focused test.
Exercise the real session runtime against a stub SDK client: a
handed-over row keeps its id and gains the created session's native
ref, while a call without one still derives a session-keyed id.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
3d59a4a9acWout Stiens committed on 9/1/2026, 5:43:06 AMparent47cc616
1 file changedLine totals unavailable