getMockImplementation only returns permanent implementation, not onceImplementation

[chaptergy]

Describe the bug

The getMockImplementation() method only returns permanent implementations, not implementations only added once. So the return value of getMockImplementation() can be different, than the implementation being called.
To me, it would make sense, that this method always returns the implementation, that will be called on the next call.

Reproduction

const fn = vi.fn();
fn.mockReturnValueOnce('foo');
const mockImpl = fn.getMockImplementation(); // This returns undefined

System Info

Binaries:
    Node: 20.15.1
    npm: 10.9.0
  npmPackages:
    vite: ^5.4.6 => 5.4.6
    vitest: ^2.1.1 => 2.1.1

Used Package Manager

npm

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• The provided reproduction is a minimal reproducible example of the bug.