fix: let returns override returnsArg

[pupuking723]

Fixes #2656.

This clears the previous returnsArg() behavior when returns() is configured afterwards. Without clearing returnArgAt, the behavior invocation path still prioritizes returnsArg over the explicit return value, so stub.returnsArg(0).returns(42) returns the first argument instead of 42.

The added regression test covers that chaining order directly.

Verification:

• npm run test-node -- test/src/stub-test.js --grep "is superseded by returns"
• npm run test-node -- test/src/stub-test.js --grep "returnsArg"
• npm run test-node -- test/src/stub-test.js
• npm run test-node
• npx eslint src/sinon/default-behaviors.js test/src/stub-test.js --max-warnings 0
• npx prettier --check src/sinon/default-behaviors.js test/src/stub-test.js