Fix mixed string type foreign key constraints

Fixes dolthub/dolt#9494

MySQL allows mixed string types in foreign key constraints:
- CHAR can reference VARCHAR and vice versa
- BINARY can reference VARBINARY and vice versa

This change updates the foreignKeyComparableTypes function to allow these
mixed type combinations by adding a new compatibleStringTypes helper function
that follows existing codebase naming patterns and includes proper nil safety checks.

The fix enables 6 previously skipped tests that validate the mixed string
type functionality.

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

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

Author: elianddb

enginetest/queries/foreign_key_queries.go

@@ -2807,21 +2807,18 @@ var CreateForeignKeyTests = []ScriptTest{
 				ExpectedErrStr: "string 'abc' is too large for column 'c'",
 			},
 			{
-				Skip:  true,
 				Query: "create table child_varchar_10 (vc varchar(10), foreign key (vc) references parent(c));",
 				Expected: []sql.Row{
 					{types.NewOkResult(0)},
 				},
 			},
 			{
-				Skip:  true,
 				Query: "insert into child_varchar_10 values ('abc');",
 				Expected: []sql.Row{
 					{types.NewOkResult(1)},
 				},
 			},
 			{
-				Skip:        true,
 				Query:       "insert into child_varchar_10 values ('abcdefghij');",
 				ExpectedErr: sql.ErrForeignKeyChildViolation,
 			},
@@ -2850,21 +2847,18 @@ var CreateForeignKeyTests = []ScriptTest{
 				ExpectedErrStr: "string 'abc' is too large for column 'b'",
 			},
 			{
-				Skip:  true,
 				Query: "create table child_varbinary_10 (vb varbinary(10), foreign key (vb) references parent(b));",
 				Expected: []sql.Row{
 					{types.NewOkResult(0)},
 				},
 			},
 			{
-				Skip:  true,
 				Query: "insert into child_varbinary_10 values ('abc');",
 				Expected: []sql.Row{
 					{types.NewOkResult(1)},
 				},
 			},
 			{
-				Skip:        true,
 				Query:       "insert into child_varbinary_10 values ('abcdefghij');",
 				ExpectedErr: sql.ErrForeignKeyChildViolation,
 			},

sql/plan/alter_foreign_key.go

@@ -670,12 +670,48 @@ func foreignKeyComparableTypes(ctx *sql.Context, type1 sql.Type, type2 sql.Type)
 				return false
 			}
 		} else {
-			return false
+			// MySQL allows mixed string types in foreign key constraints:
+			// - CHAR can reference VARCHAR and vice versa
+			// - BINARY can reference VARBINARY and vice versa
+			if compatibleStringTypes(type1, type2) {
+				type1String := type1.(sql.StringType)
+				type2String := type2.(sql.StringType)
+				if type1String.Collation().CharacterSet() != type2String.Collation().CharacterSet() {
+					return false
+				}
+			} else {
+				return false
+			}
 		}
 	}
 	return true
 }
 
+// compatibleStringTypes checks if two different string types are compatible for foreign key constraints.
+// MySQL allows mixed string types within the same category:
+// - Character types: CHAR and VARCHAR
+// - Binary types: BINARY and VARBINARY
+func compatibleStringTypes(type1 sql.Type, type2 sql.Type) bool {
+	if type1 == nil || type2 == nil {
+		return false
+	}
+	
+	t1 := type1.Type()
+	t2 := type2.Type()
+	
+	// Check if both are character string types (CHAR/VARCHAR)
+	if (t1 == sqltypes.Char || t1 == sqltypes.VarChar) && (t2 == sqltypes.Char || t2 == sqltypes.VarChar) {
+		return true
+	}
+	
+	// Check if both are binary string types (BINARY/VARBINARY)
+	if (t1 == sqltypes.Binary || t1 == sqltypes.VarBinary) && (t2 == sqltypes.Binary || t2 == sqltypes.VarBinary) {
+		return true
+	}
+	
+	return false
+}
+
 // exprsAreIndexPrefix returns whether the given expressions are a prefix of the given index expressions
 func exprsAreIndexPrefix(exprs, indexExprs []string) bool {
 	if len(exprs) > len(indexExprs) {