fix: nextResult and nextError return the spy, so they chain

[shaurya703]

Closes #60

Spy declares nextResult and nextError as returning this, but populateSpy returned the internal state object, which has neither method. So the chain the types allow compiles and then throws at runtime:

const s = spy(() => 'real')
s.nextResult('a').nextResult('b')
// TypeError: s.nextResult(...).nextResult is not a function

This returns the spy from both, the same way willCall in internalSpyOn already does. The two existing call sites in the tests call them as separate statements, so nothing else changes.

Test

One new test pins the identity (nextResult(...) and nextError(...) both toBe the spy) and a mixed chain nextResult(2).nextError(err).nextResult(3), then drains the queue in order. It fails on main with expected { reset: [Function reset], …(7) } to be [Function spy] and passes with the change.

pnpm build && pnpm test: 53/53. tsc --noEmit clean.