Improve HighSeas validation and fix HighScore duplicate‑UUID logic

[JaskRendix]

HighScore

• fixed checkHighScore to correctly detect duplicate game UUIDs and update an existing entry only when the new score is higher
• corrected an indexing bug caused by reverse iteration, ensuring the method returns the proper replacement index
• fixed tidyScores sorting and truncation bug by ensuring scores are explicitly sorted in descending order before applying .subList(NUMBER_OF_HIGH_SCORES, scores.size()).clear(), resolving issues where excess scores bypassed retention limits

Test Coverage Improvements

• Expanded and strengthened test coverage for: HighSeasTest, HighScoreTest, EuropeTest
• Added a new test suite: ServerEuropeTest

Next Steps / Post-Merge Todo:
Once this PR is merged, open a follow-up PR to clean up src/net/sf/freecol/server/control/InGameController.java by replacing this manual loop check:

            boolean hasMap = false;
            for (Location dest : destinations) {
                if (dest instanceof Map) {
                    hasMap = true;
                    break;
                }
            }
            if (!hasMap) {
                Map map = getGame().getMap();
                destinations.add(map);
            }

with the new helper method:

            highSeas.ensureMapDestination();