Core/Players: Prevent creation of characters with duplicate names

Closes #30299

(cherry picked from commit 49ab2bbe38)

sql/updates/characters/cata_classic/2024_10_03_00_characters.sql

@@ -0,0 +1,29 @@
+-- append guid as hex to characters who have duplicate names in database except the one with lowest guid (assumed to have been created first)
+-- and flag them for rename
+CREATE TEMPORARY TABLE `characters_to_rename` SELECT
+    c2.`guid`
+  FROM
+    (
+      SELECT
+        c1.`name` AS `name`,
+        MIN(c1.`guid`) AS originalGuid
+      FROM
+        `characters` c1 
+      WHERE
+        LENGTH(c1.`name`) > 0 
+      GROUP BY
+        1 
+      HAVING
+        COUNT(*) > 1
+    ) c3
+    INNER JOIN `characters` c2 ON c3.`name` = c2.`name`
+  WHERE
+    c2.guid <> c3.originalGuid;
+
+UPDATE `characters` SET `name` = CONCAT(SUBSTRING(`name` FROM 1 FOR 12 - LENGTH(CONV(`guid`, 10, 16))), CONV(`guid`, 10, 16)), `at_login` = `at_login` | 1 WHERE `guid` IN (SELECT `guid` FROM `characters_to_rename`);
+
+-- recreate name index with unique constraint
+ALTER TABLE `characters` DROP INDEX `idx_name`;
+ALTER TABLE `characters` MODIFY COLUMN `name` varchar(12) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NULL AFTER `account`;
+UPDATE `characters` SET `name` = NULL WHERE `name` = '';
+ALTER TABLE `characters` ADD UNIQUE INDEX `idx_name` (`name` ASC);