Upgrade vitest to v1 and fix test definitions

This commit upgrades the `vitest` library to its first major version
(v1) resolving issues with previously unexecuted tests due to improperly
nested `it` blocks.

The migration to v1 uncovered error messages indicating the misuse of
`it` blocks, as described in vitest-dev/vitest#4229 and
vitest-dev/vitest#4262, prompting a restructuring of test cases for
proper execution.

Additionally, this commit adjusts singleton test definitions in
`DependencyProvider.spec.ts` to better reflect real usage scenarios and
correctly implement singleton pattern tests, enhancing test reliability.

Changes:

- Upgrade `vitest` from v0 to v1.
- Correct test definitions by organizing `it` blocks within `describe`
  blocks.
- Fix singleton test definition in `DependencyProvider.spec.ts`.