feat(writer): 🎉 Implement add column statements for schema evolution

* Add functionality to generate ALTER TABLE ADD COLUMN statements for existing tables.
* Introduce tests for generating and writing add column statements.
* Enhance schema evolution capabilities when new columns are added.

pkg/writers/pgsql/writer_test.go

@@ -438,3 +438,130 @@ func TestColumnSizeSpecifiers(t *testing.T) {
 		}
 	}
 }
+
+func TestGenerateAddColumnStatements(t *testing.T) {
+	// Create a test database with tables that have new columns
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+
+	// Create a table with columns
+	table := models.InitTable("users", "public")
+
+	// Existing column
+	idCol := models.InitColumn("id", "users", "public")
+	idCol.Type = "integer"
+	idCol.NotNull = true
+	idCol.Sequence = 1
+	table.Columns["id"] = idCol
+
+	// New column to be added
+	emailCol := models.InitColumn("email", "users", "public")
+	emailCol.Type = "varchar"
+	emailCol.Length = 255
+	emailCol.NotNull = true
+	emailCol.Sequence = 2
+	table.Columns["email"] = emailCol
+
+	// New column with default
+	statusCol := models.InitColumn("status", "users", "public")
+	statusCol.Type = "text"
+	statusCol.Default = "active"
+	statusCol.Sequence = 3
+	table.Columns["status"] = statusCol
+
+	schema.Tables = append(schema.Tables, table)
+	db.Schemas = append(db.Schemas, schema)
+
+	// Create writer
+	options := &writers.WriterOptions{}
+	writer := NewWriter(options)
+
+	// Generate ADD COLUMN statements
+	statements, err := writer.GenerateAddColumnsForDatabase(db)
+	if err != nil {
+		t.Fatalf("GenerateAddColumnsForDatabase failed: %v", err)
+	}
+
+	// Join all statements to verify content
+	output := strings.Join(statements, "\n")
+	t.Logf("Generated ADD COLUMN statements:\n%s", output)
+
+	// Verify expected elements
+	expectedStrings := []string{
+		"ALTER TABLE public.users ADD COLUMN id integer NOT NULL",
+		"ALTER TABLE public.users ADD COLUMN email varchar(255) NOT NULL",
+		"ALTER TABLE public.users ADD COLUMN status text DEFAULT 'active'",
+		"information_schema.columns",
+		"table_schema = 'public'",
+		"table_name = 'users'",
+		"column_name = 'id'",
+		"column_name = 'email'",
+		"column_name = 'status'",
+	}
+
+	for _, expected := range expectedStrings {
+		if !strings.Contains(output, expected) {
+			t.Errorf("Output missing expected string: %s\nFull output:\n%s", expected, output)
+		}
+	}
+
+	// Verify DO blocks are present for conditional adds
+	doBlockCount := strings.Count(output, "DO $$")
+	if doBlockCount < 3 {
+		t.Errorf("Expected at least 3 DO blocks (one per column), got %d", doBlockCount)
+	}
+
+	// Verify IF NOT EXISTS logic
+	ifNotExistsCount := strings.Count(output, "IF NOT EXISTS")
+	if ifNotExistsCount < 3 {
+		t.Errorf("Expected at least 3 IF NOT EXISTS checks (one per column), got %d", ifNotExistsCount)
+	}
+}
+
+func TestWriteAddColumnStatements(t *testing.T) {
+	// Create a test database
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+
+	// Create a table with a new column to be added
+	table := models.InitTable("products", "public")
+
+	idCol := models.InitColumn("id", "products", "public")
+	idCol.Type = "integer"
+	table.Columns["id"] = idCol
+
+	// New column with various properties
+	descCol := models.InitColumn("description", "products", "public")
+	descCol.Type = "text"
+	descCol.NotNull = false
+	table.Columns["description"] = descCol
+
+	schema.Tables = append(schema.Tables, table)
+	db.Schemas = append(db.Schemas, schema)
+
+	// Create writer with output to buffer
+	var buf bytes.Buffer
+	options := &writers.WriterOptions{}
+	writer := NewWriter(options)
+	writer.writer = &buf
+
+	// Write ADD COLUMN statements
+	err := writer.WriteAddColumnStatements(db)
+	if err != nil {
+		t.Fatalf("WriteAddColumnStatements failed: %v", err)
+	}
+
+	output := buf.String()
+	t.Logf("Generated output:\n%s", output)
+
+	// Verify output contains expected elements
+	if !strings.Contains(output, "ALTER TABLE public.products ADD COLUMN id integer") {
+		t.Errorf("Output missing ADD COLUMN for id\nFull output:\n%s", output)
+	}
+	if !strings.Contains(output, "ALTER TABLE public.products ADD COLUMN description text") {
+		t.Errorf("Output missing ADD COLUMN for description\nFull output:\n%s", output)
+	}
+	if !strings.Contains(output, "DO $$") {
+		t.Errorf("Output missing DO block\nFull output:\n%s", output)
+	}
+}