Write a Python function to calculate the covariance matrix for a given set of vectors. The function should take a list of lists, where each inner list represents a feature with its observations, and return a covariance matrix as a list of lists. Additionally, provide test cases to verify the correctness of your implementation. Example: Input: [[1, 2, 3], [4, 5, 6]] Output: [[1.0, 1.0], [1.0, 1.0]] Reasoning: The covariance between the two features is calculated based on their deviations from the mean. For the given vectors, both covariances are 1.0, resulting in a symmetric covariance matrix. def calculate_covariance_matrix(vectors: list[list[float]]) -> list[list[float]]: # Your code here return []