fix: ensure last behavior setter always wins

[ordinary9843]

Problem

When chaining stub behavior setters, the last call should always win. However, several combinations silently failed because flags set by earlier setters weren't cleared.

For example:

const stub = sinon.stub();
stub.returnsArg(0);
stub.returns(42); // should win, but doesn't — returnsArg silently persists
stub('ignored'); // returns 'ignored' instead of 42

This happens because behavior.invoke() checks returnArgAt (priority 2) before returnValue (priority 12). When returnsArg is called first, it sets returnArgAt. When returns is called next, it sets returnValue but leaves returnArgAt intact — so returnArgAt still takes effect.

The same issue affects returnsThis, throwsArg, and any setter that doesn't clear competing state.

Fix

Add a resetReturnBehavior() helper that clears all mutually exclusive return-behavior flags. Call it at the start of each setter so the most recently called setter always takes effect, regardless of the priority order in invoke().

Broken combinations fixed

Earlier setter	Later setter	Before	After
returnsArg	returns	broken	fixed
returnsThis	returns	broken	fixed
throwsArg	returns	broken	fixed
returnsArg	throwsArg	broken	fixed
returnsThis	throwsArg	broken	fixed
returnsArg	returnsThis	broken	fixed

Fixes #2656

[fatso83]

Thanks for this, could you rebase on the latest main and I'll have a new look?

[fatso83]

I think some of these changes will be made redundant by #2670?

[ordinary9843]

Rebased on latest main. The test suite in test/src/ passes cleanly — the failures in test/issues/ are due to the ESM migration in #2683 (the issues test still uses require against the built lib/).

On the overlap with #2670: the two address different call orderings. #2670 cleans up setter state when callThrough() is the last call; this PR clears all competing flags when any setter is the last call (e.g. returnsArg then returns). Both are needed for consistent last-setter-wins behavior.

Also worth noting — after #2683, lib/ is a build artifact. #2670 currently targets lib/sinon/default-behaviors.js directly, so it will need updating before it can land.