test: add validate_search_result boundary tests (DOC-FULL-025)

- 10 new boundary tests: empty snippet, match at end, start>end,
  exact bounds, query longer than snippet, unicode snippet,
  zero-length match, match_end==snippet_len, query mismatch
- Total 14 validate_search_result tests + 28 other search tests

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/zx_document_core/src/search.rs

@@ -433,4 +433,89 @@ mod tests {
         };
         assert!(!validate_search_result(&result, "test"));
     }
+
+    #[test]
+    fn test_validate_search_result_empty_snippet() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: String::new(), match_start: 0, match_end: 0,
+        };
+        assert!(!validate_search_result(&result, "hello"));
+    }
+
+    #[test]
+    fn test_validate_search_result_match_at_snippet_end() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "the end".into(), match_start: 4, match_end: 7,
+        };
+        assert!(validate_search_result(&result, "end"));
+    }
+
+    #[test]
+    fn test_validate_search_result_start_gt_end() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "text".into(), match_start: 3, match_end: 1,
+        };
+        assert!(!validate_search_result(&result, "test"));
+    }
+
+    #[test]
+    fn test_validate_search_result_exact_bounds() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "exact".into(), match_start: 0, match_end: 5,
+        };
+        assert!(validate_search_result(&result, "exact"));
+    }
+
+    #[test]
+    fn test_validate_search_result_query_longer_than_snippet() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "hi".into(), match_start: 0, match_end: 2,
+        };
+        assert!(!validate_search_result(&result, "hello"));
+        // match_end (2) == snippet.len() so bounds ok, but "hi" != "hello"
+    }
+
+    #[test]
+    fn test_validate_search_result_unicode_snippet() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "你好世界".into(), match_start: 0, match_end: 6, // "你好" = 6 bytes in UTF-8
+        };
+        assert!(validate_search_result(&result, "你好"));
+    }
+
+    #[test]
+    fn test_validate_search_result_zero_length_match() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "test".into(), match_start: 1, match_end: 1,
+        };
+        // Zero-length match: start==end, query is empty string
+        // Empty query → early return false
+        assert!(!validate_search_result(&result, ""));
+    }
+
+    #[test]
+    fn test_validate_search_result_match_end_equals_snippet_len() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "match".into(), match_start: 0, match_end: 5,
+        };
+        // match_end == snippet.len() is valid
+        assert!(validate_search_result(&result, "match"));
+    }
+
+    #[test]
+    fn test_validate_search_result_query_mismatch() {
+        let result = SearchResult {
+            block_id: "b1".into(), line_number: None, page_number: None, chapter_id: None,
+            snippet: "hello world".into(), match_start: 0, match_end: 5,
+        };
+        assert!(!validate_search_result(&result, "world"));
+    }
 }