Extract PostgreSQL version constants (#181)

- Extract magic number into a structured constant
- Add documentation with links to PostgreSQL release notes
- Add comprehensive tests for version checking

This refactoring improves code readability and makes it easier to add
support for future PostgreSQL versions.

Author: teoljungberg

lib/fx/adapters/postgres/connection.rb

@@ -10,15 +10,23 @@ class Postgres
       #
       # @api private
       class Connection < SimpleDelegator
-        # https://www.postgresql.org/docs/9.6/sql-dropfunction.html
-        # https://www.postgresql.org/docs/10/sql-dropfunction.html
+        # PostgreSQL version constants for feature support
+        POSTGRES_VERSIONS = {
+          # PostgreSQL 10.0 - introduced DROP FUNCTION without args
+          # https://www.postgresql.org/docs/10/sql-dropfunction.html
+          v10: 10_00_00
+        }.freeze
+
         def support_drop_function_without_args
-          pg_connection = undecorated_connection.raw_connection
-          pg_connection.server_version >= 10_00_00
+          server_version >= POSTGRES_VERSIONS[:v10]
         end
 
         private
 
+        def server_version
+          undecorated_connection.raw_connection.server_version
+        end
+
         def undecorated_connection
           __getobj__
         end

spec/fx/adapters/postgres_spec.rb

@@ -141,4 +141,46 @@
       expect(adapter.triggers.map(&:name)).to eq ["uppercase_users_name"]
     end
   end
+
+  describe "#support_drop_function_without_args" do
+    it "returns true for PostgreSQL version 10.0" do
+      adapter = Fx::Adapters::Postgres.new
+      connection = adapter.send(:connection)
+      allow(connection).to receive(:server_version).and_return(10_00_00)
+
+      result = connection.support_drop_function_without_args
+
+      expect(result).to be(true)
+    end
+
+    it "returns true for PostgreSQL version 11.0" do
+      adapter = Fx::Adapters::Postgres.new
+      connection = adapter.send(:connection)
+      allow(connection).to receive(:server_version).and_return(11_00_00)
+
+      result = connection.support_drop_function_without_args
+
+      expect(result).to be(true)
+    end
+
+    it "returns false for PostgreSQL version 9.6" do
+      adapter = Fx::Adapters::Postgres.new
+      connection = adapter.send(:connection)
+      allow(connection).to receive(:server_version).and_return(9_06_00)
+
+      result = connection.support_drop_function_without_args
+
+      expect(result).to be(false)
+    end
+
+    it "returns false for PostgreSQL version 9.5" do
+      adapter = Fx::Adapters::Postgres.new
+      connection = adapter.send(:connection)
+      allow(connection).to receive(:server_version).and_return(9_05_00)
+
+      result = connection.support_drop_function_without_args
+
+      expect(result).to be(false)
+    end
+  end
 end