[CodeQuality] Add WillReturnCallbackFallbackToReturnFalseRector

[TomasVotruba]

Adds a code quality rule that appends a return false; fallback to a willReturnCallback() closure that only returns booleans but can fall through without a return.

Before:

$eventMock->expects($matcher)->method('checkContext')->willReturnCallback(function (...$parameters) use ($matcher) {
    if (1 === $matcher->numberOfInvocations()) {
        return true;
    }
    if (2 === $matcher->numberOfInvocations()) {
        return false;
    }
});

After:

$eventMock->expects($matcher)->method('checkContext')->willReturnCallback(function (...$parameters) use ($matcher) {
    if (1 === $matcher->numberOfInvocations()) {
        return true;
    }
    if (2 === $matcher->numberOfInvocations()) {
        return false;
    }

    return false;
});

Rule triggers only when every explicit return in the closure returns true/false and the body does not already end with a return. Registered in the phpunit-code-quality set.

Note: sibling WillReturnCallbackFallbackToThrowRector covers the same missing-fallback shape but throws; it is deprecated. This one targets the common bool-returning mock case with a concrete default.