log_mix(container, container) should check first argument is simplex

## Description

The function log_mix taking two collections does not check that the first container is a simplex.

## Expected Output

Rejection when the first argument is not a simplex, e.g.,

```stan
target += log_mix({0.5, 0.9}, {-1.2, -4.2});
```

#### Current Version:
v4.9.0
