chore(imports): test/unit/engine-coverage-script.test.ts drifted from #9221's extension convention (introduced by #9245)

[JSONbored]

Context

#9221 added npm run import-specifiers:check (wired into test:ci) to keep relative-import specifiers uniform per resolution zone. #9245 ("close four testing/coverage-gate blind spots") merged with a .js-suffixed relative specifier in test/unit/engine-coverage-script.test.ts, which is in the Bundler-resolved zone (src/**/scripts/**/test/**) and should be extensionless:

$ npm run import-specifiers:check
  test/unit/engine-coverage-script.test.ts: "../../scripts/engine-coverage.js" — this file's zone (src/scripts/test) resolves extensionless under Bundler

Same class of gap as #9240 (a PR merging with a branch state that predates or otherwise missed the drift check).

Requirements

1. Drop the .js extension from the flagged specifier (and any other drift import-specifiers:check still reports by the time this is picked up).
2. npm run import-specifiers:check exits 0 afterward.

Expected outcome

main is clean against its own drift guard again.

[linear-code]

JSO-6422