Fix frozen string literal warnings

Use +\"\" for string literals that are mutated with << operator.
This prepares for Ruby's future frozen string literal default.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yahonda

lib/plsql/package.rb

@@ -56,7 +56,7 @@ def [](object_name)
     private
 
       def method_missing(method, *args, &block)
-        method = method.to_s
+        method = +method.to_s
         method.chop! if (assignment = method[/=$/])
 
         case (object = self[method])

lib/plsql/procedure.rb

@@ -315,7 +315,7 @@ def construct_argument_list_for_overloads # :nodoc:
     def ensure_tmp_tables_created(overload) # :nodoc:
       return if @tmp_tables_created.nil? || @tmp_tables_created[overload]
       @tmp_table_names[overload] && @tmp_table_names[overload].each do |table_name, argument_metadata|
-        sql = "CREATE GLOBAL TEMPORARY TABLE #{table_name} (\n"
+        sql = +"CREATE GLOBAL TEMPORARY TABLE #{table_name} (\n"
         element_metadata = argument_metadata[:element]
         case element_metadata[:data_type]
         when "PL/SQL RECORD"

lib/plsql/procedure_call.rb

@@ -149,10 +149,10 @@ def matching_oracle_types_for_ruby_value(value)
       end
 
       def construct_sql(args)
-        @declare_sql = ""
-        @assignment_sql = ""
-        @call_sql = ""
-        @return_sql = ""
+        @declare_sql = +""
+        @assignment_sql = +""
+        @call_sql = +""
+        @return_sql = +""
         @return_vars = []
         @return_vars_metadata = {}
 
@@ -218,7 +218,7 @@ def construct_sql(args)
         end
         add_out_variables
 
-        @sql = @declare_sql.empty? ? "" : "DECLARE\n" << @declare_sql
+        @sql = @declare_sql.empty? ? +"" : +"DECLARE\n" << @declare_sql
         @sql << "BEGIN\n" << @assignment_sql << dbms_output_enable_sql << @call_sql << @return_sql << "END;\n"
       end
 
@@ -326,7 +326,7 @@ def add_record_declaration(argument, argument_metadata)
           "l_#{argument} #{argument_metadata[:sql_type_name]};\n"
         else
           fields_metadata = argument_metadata[:fields]
-          sql = "TYPE t_#{argument} IS RECORD (\n"
+          sql = +"TYPE t_#{argument} IS RECORD (\n"
           sql << record_fields_sorted_by_position(fields_metadata).map do |field|
             metadata = fields_metadata[field]
             "#{field} #{type_to_sql(metadata)}"
@@ -341,7 +341,7 @@ def record_fields_sorted_by_position(fields_metadata)
       end
 
       def record_assignment_sql_values_metadata(argument, argument_metadata, record_value)
-        sql = ""
+        sql = +""
         bind_values = {}
         bind_metadata = {}
         (record_value || {}).each do |key, value|

lib/plsql/schema.rb

@@ -234,7 +234,7 @@ def find_database_object(name, override_schema_name = nil)
       end
 
       def _errors(object_schema_name, object_name, object_type)
-        result = ""
+        result = +""
         previous_line = 0
         select_all(
           "SELECT e.line, e.position, e.text error_text, s.text source_text

lib/plsql/table.rb

@@ -87,9 +87,9 @@ def column_names
     def select(first_or_all, sql_params = "", *bindvars)
       case first_or_all
       when :first, :all
-        select_sql = "SELECT * "
+        select_sql = +"SELECT * "
       when :count
-        select_sql = "SELECT COUNT(*) "
+        select_sql = +"SELECT COUNT(*) "
       else
         raise ArgumentError, "Only :first, :all or :count are supported"
       end