next.js

e9df6ed9 - Add instant validation tests for parallel slot config attribution (#91656)

Add instant validation tests for parallel slot config attribution (#91656) Tests that unstable_instant configs placed at different positions across a parallel route fork correctly attribute errors to the blocking segment, with the cause pointing to the config that requires instant behavior. Includes both invalid cases that produce errors and valid cases that pass cleanly. Does not yet detect incompatible configurations like false on one slot alongside an instant config on a sibling — that will be handled in a followup.