Apply Intellij code cleanup suggestions

[NolwennD]

Hello,

I apply Intellij code cleanup suggestion without behavior modifications.

Most of modifications are:

• Use the diamond operator (<>) instead of explicit generic type arguments.
• Replace lambdas with method references (e.g. this::formatTerm, Optional::isPresent, Stream::of).
• Replace indexed for loops with enhanced for loops where the index was only used for get(i).
• Make fields final where they are never reassigned.
• Pass the source collection to the copy constructor (new ArrayList<>(other)) instead of looping or calling addAll.
• Use Optional idioms: isEmpty() instead of !isPresent(), ifPresent(...) instead of if (isPresent()) { get() }, and Optional.stream().