refactor!: remove original impl workaround for Vitest 3

[mcous]

Since Vitest v4 has now been released (and Vitest v5 is on the way) this PR removes the workaround mockReset in Vitest v3 added in #28. The fix moving forward for users is to upgrade to Vitest >= v4

BREAKING CHANGE: When using Vitest v3, the mock implementation fallback may not work properly after a mockReset. Upgrade to Vitest v4 to resolve

[mcous]

Changed my mind on this one, I think the hack is still valuable as long as we support vitest v3

[mcous]

I keep flipping back and forth here

Keep it

• Pro: Prevents silent breakage for v3 users who follow vitest-when's own mockReset recommendation with vi.fn(() => impl). Failure mode is confusing — looks like a vitest-when bug, not a Vitest bug.
• Con: Introduces unintended behavior — vi.spyOn mocks fall back to the real implementation. This silently disappears on v4, making the hack itself a source of breaking changes across Vitest upgrades.
  • See Original implementation does not get called on vitest@v4 when using vi.spyOn #41

Remove it

• Pro: Eliminates the unintended spyOn fallback. Cleaner, smaller code. Unintended behavior can't be preserved on v4 anyway, so carrying it in v3 only creates a hidden migration trap.
• Con: v3 users following the docs (mockReset + initial implementation) hit a confusing failure that looks like a vitest-when regression
  • Mitigation: changelog note pointing to Vitest v4 upgrade.

Conclusion

My gut says to remove the hack for v1 - it's less code to maintain and removes a source of unintended behavior