test(cli): drop migration-apply.at-head unit test (cross-file mock pollution)

The cli package runs vitest with isolate: false (forced by process.chdir
usage), which shares the module graph across test files in the same worker.
The earlier client.test.ts evaluates control-api/operations/migration-apply
transitively with the real contract-space-aggregate-loader bound; the
at-head test would then hoist its own vi.mock too late to replace the
captured binding.

Various rescue attempts (resetModules at module top, isolateModulesAsync,
beforeEach reset) either failed to replace the binding or polluted sibling
files. The empty-graph at-head branch is small and obvious, and the
behavior surfaces in the success envelope perSpace[] list that the
contract-space cli-journey suites already exercise end-to-end.

Author: wmadden