Maintenance: CrossEncoderReranker - Add comprehensive tests and improve code quality

This commit adds comprehensive test coverage for CrossEncoderReranker
implementations and fixes a code quality issue in NoopCrossEncoderReranker.

Changes made:
- Replace deprecated Collectors.toList() with .toList() in NoopCrossEncoderReranker
- Add 11 comprehensive unit tests for NoopCrossEncoderReranker covering:
  * Score-based sorting (descending order)
  * Equal scores handling
  * Edge cases (empty list, single candidate, null/blank query)
  * Negative and zero scores
  * Large score values
  * Many candidates
- Add 15 comprehensive unit tests for NaverCrossEncoderReranker covering:
  * Successful API reranking
  * Partial document citation
  * Fallback scenarios (no citations, API errors, invalid JSON)
  * Null/blank query handling
  * Empty candidates handling
  * API key validation
  * Request body and headers verification
  * Document structure validation

All tests pass successfully and maintain consistency with existing test patterns
in the codebase.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: github-actions[bot]

src/main/java/spring/memewikibe/infrastructure/ai/NoopCrossEncoderReranker.java

@@ -4,7 +4,6 @@
 
 import java.util.Comparator;
 import java.util.List;
-import java.util.stream.Collectors;
 
 /**
  * Default no-op Cross-Encoder reranker that preserves prior order by score.
@@ -16,6 +15,6 @@ public List<Long> rerank(String query, List<Candidate> candidates) {
         return candidates.stream()
             .sorted(Comparator.comparingDouble(Candidate::priorScore).reversed())
             .map(Candidate::id)
-            .collect(Collectors.toList());
+            .toList();
     }
 }