📝 Description
Implement FMultiMap, a persistent, immutable multi-map where a key is associated with multiple values.

✅ Expected Behavior
put(K key, V value) → Returns a new FMultiMap with value added under key.
remove(K key, V value) → Returns a new FMultiMap without value under key.
get(K key) → Returns a functional collection (e.g., FList) of values associated with key.
containsKey(K key) → Returns true if key exists.
containsEntry(K key, V value) → Returns true if value exists under key.
📌 Requirements
Implement FMultiMap<K, V> as an immutable data structure.
Store values as functional collections (e.g., FList or FSet).
Write unit tests covering all methods.
Provide proper Javadocs for all public methods.