fix(search): column bulk operations search not returning results at scale

When searching by column name pattern (e.g., "MAT") in column bulk
operations, the composite aggregation returned ALL column names from
matching documents, then post-filtered in Java. With 20000+ columns,
the first composite page of 25 names rarely contained matches, so
users saw 0 results.

Switch to terms aggregation with `include` regex when a search pattern
is set. This filters at the ES/OS aggregation level — only matching
column names produce buckets. Two-phase approach: (1) lightweight
names query to get all matching names + accurate total, (2) targeted
data query with top_hits for the current page only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sonika-shah

openmetadata-service/src/main/java/org/openmetadata/service/search/ColumnAggregator.java

@@ -21,6 +21,28 @@ public interface ColumnAggregator {
 
   ColumnGridResponse aggregateColumns(ColumnAggregationRequest request) throws IOException;
 
+  /**
+   * Convert a plain text pattern to a case-insensitive regex for ES/OS terms include. Lucene regex
+   * does not support (?i), so each letter is expanded to a character class: "MAT" → [mM][aA][tT].
+   */
+  static String toCaseInsensitiveRegex(String pattern) {
+    StringBuilder sb = new StringBuilder(".*");
+    for (char c : pattern.toCharArray()) {
+      if (Character.isLetter(c)) {
+        sb.append('[')
+            .append(Character.toLowerCase(c))
+            .append(Character.toUpperCase(c))
+            .append(']');
+      } else if (".+*?|[](){}^$\\~@&#<>\"".indexOf(c) >= 0) {
+        sb.append('\\').append(c);
+      } else {
+        sb.append(c);
+      }
+    }
+    sb.append(".*");
+    return sb.toString();
+  }
+
   class ColumnAggregationRequest {
     private int size = 1000;
     private String cursor;