Update charset validation tests to match MySQL error format

- Updated script_queries.go to expect MySQL-compatible error messages
- Updated enginetests.go prepared statement tests to expect MySQL-compatible format
- Changed from: "invalid string for charset utf8mb4: '[152 118 84 50 16]'"
- Changed to: "Incorrect string value: '\x98' for column 'c' at row 1"
- Tests now expect the correct MySQL error format as implemented in fix for #8893

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

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

Author: elianddb

enginetest/enginetests.go

@@ -4501,14 +4501,14 @@ func TestPreparedInsert(t *testing.T, harness Harness) {
 					Bindings: map[string]sqlparser.Expr{
 						"v1": mustBuildBindVariable([]byte{0x99, 0x98, 0x97}),
 					},
-					ExpectedErrStr: "invalid string for charset utf8mb4: '[153 152 151]'",
+					ExpectedErrStr: "Incorrect string value: '\\x99' for column 'v1' at row 1",
 				},
 				{
 					Query: "INSERT INTO test VALUES (?);",
 					Bindings: map[string]sqlparser.Expr{
 						"v1": mustBuildBindVariable(string([]byte{0x99, 0x98, 0x97})),
 					},
-					ExpectedErrStr: "invalid string for charset utf8mb4: '[153 152 151]'",
+					ExpectedErrStr: "Incorrect string value: '\\x99' for column 'v1' at row 1",
 				},
 				{
 					Query: "INSERT INTO test2 VALUES (?);",

enginetest/queries/script_queries.go

@@ -7532,15 +7532,15 @@ where
 		Assertions: []ScriptTestAssertion{
 			{
 				Query:          "insert into t(c) values (X'9876543210');",
-				ExpectedErrStr: "invalid string for charset utf8mb4: '[152 118 84 50 16]'",
+				ExpectedErrStr: "Incorrect string value: '\\x98' for column 'c' at row 1",
 			},
 			{
 				Query:          "insert into t(v) values (X'9876543210');",
-				ExpectedErrStr: "invalid string for charset utf8mb4: '[152 118 84 50 16]'",
+				ExpectedErrStr: "Incorrect string value: '\\x98' for column 'v' at row 1",
 			},
 			{
 				Query:          "insert into t(txt) values (X'9876543210');",
-				ExpectedErrStr: "invalid string for charset utf8mb4: '[152 118 84 50 16]'",
+				ExpectedErrStr: "Incorrect string value: '\\x98' for column 'txt' at row 1",
 			},
 			{
 				Query: "insert into t(b) values (X'9876543210');",