docs: add Javadoc to all four DP classes

[bushidocodes]

Summary

Adds consistent Javadoc coverage across all four source files:

• Class-level docs on all four classes — describes the problem, the DP approach, and the key state representation
• CoinPickingGame.maxScore — @param, @return, @throws
• MinimumStepsChessKnight.minSteps — @param for each of the five arguments, @return (including the -1 sentinel), @throws
• HouseRobbing and WildcardPatternMatching already had method-level docs from their initial implementation; this PR adds the missing class-level docs and makes the style uniform

Private methods are left without Javadoc — their names and the class-level description are sufficient context.

All 46 tests pass locally.

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