Fix test failures and auto-increment handling for empty INSERT VALUES

- Fix SHOW CREATE TABLE keyless test to expect DEFAULT NULL for nullable columns
- Fix DESCRIBE/SHOW COLUMNS tests to expect "NULL" string for Default column
- Allow auto-increment columns to work with empty VALUES () tuples
- Extend empty VALUES handling to all tuples in mixed INSERT statements
- Update test error expectations to use correct ErrColValCountMismatch type

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

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

Author: elianddb

enginetest/queries/insert_queries.go

@@ -3136,15 +3136,15 @@ var InsertBrokenScripts = []ScriptTest{
 		Assertions: []ScriptTestAssertion{
 			{
 				Query:       "insert into t(i, j) values (1, 2, 3)",
-				ExpectedErr: sql.ErrColumnCountMismatch,
+				ExpectedErr: sql.ErrColValCountMismatch,
 			},
 			{
 				Query:       "insert into t(i, j) values (1)",
-				ExpectedErr: sql.ErrColumnCountMismatch,
+				ExpectedErr: sql.ErrColValCountMismatch,
 			},
 			{
 				Query:       "insert into t(i, j) values (1, 2), ()",
-				ExpectedErr: sql.ErrColumnCountMismatch,
+				ExpectedErr: sql.ErrColValCountMismatch,
 			},
 		},
 	},

enginetest/queries/queries.go

@@ -10462,28 +10462,28 @@ var KeylessQueries = []QueryTest{
 	{
 		Query: "DESCRIBE keyless",
 		Expected: []sql.Row{
-			{"c0", "bigint", "YES", "", nil, ""},
-			{"c1", "bigint", "YES", "", nil, ""},
+			{"c0", "bigint", "YES", "", "NULL", ""},
+			{"c1", "bigint", "YES", "", "NULL", ""},
 		},
 	},
 	{
 		Query: "SHOW COLUMNS FROM keyless",
 		Expected: []sql.Row{
-			{"c0", "bigint", "YES", "", nil, ""},
-			{"c1", "bigint", "YES", "", nil, ""},
+			{"c0", "bigint", "YES", "", "NULL", ""},
+			{"c1", "bigint", "YES", "", "NULL", ""},
 		},
 	},
 	{
 		Query: "SHOW FULL COLUMNS FROM keyless",
 		Expected: []sql.Row{
-			{"c0", "bigint", nil, "YES", "", nil, "", "", ""},
-			{"c1", "bigint", nil, "YES", "", nil, "", "", ""},
+			{"c0", "bigint", nil, "YES", "", "NULL", "", "", ""},
+			{"c1", "bigint", nil, "YES", "", "NULL", "", "", ""},
 		},
 	},
 	{
 		Query: "SHOW CREATE TABLE keyless",
 		Expected: []sql.Row{
-			{"keyless", "CREATE TABLE `keyless` (\n  `c0` bigint,\n  `c1` bigint\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin"},
+			{"keyless", "CREATE TABLE `keyless` (\n  `c0` bigint DEFAULT NULL,\n  `c1` bigint DEFAULT NULL\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin"},
 		},
 	},
 }