Now that `new fn()` returns an object, the common Jest assertion `expect(new fn()).toBeInstanceOf(fn)` is reachable — but it throws `TypeError` because `JSMockFunction` never installs a `.prototype` own property, so `createSubclassStructure` falls back to `Object.prototype` and `OrdinaryHasInstance` rejects the non-object `fn.prototype`. The missing `.prototype` is pre-existing and the construct callback itself is spec-correct, so this can be a follow-up: give each `JSMockFunction` a fresh `.pro

Now that `new fn()` actually works, it might be worth pushing `thisObject` into `fn->instances` here so `fn.mock.instances[0]` is the constructed instance — that's the canonical use case for `mock.instances` in Jest. This is technically pre-existing (Bun has never populated `mock.instances` for any call), so feel free to defer to a follow-up, but flagging since the PR description says this matches how Jest mocks behave when constructed.