Refactor Mailgun config to use getEnvVar and improve checks

- Replace getEnv with getEnvVar for environment variable access - Update
MailgunConfig types to allow undefined values - Enhance
isMailgunConfigured to check for undefined and empty values - Update
isDevelopmentMode to check production status - Improve test mocks for
environment variable handling

services/__tests__/mailgun.config.test.ts

@@ -308,6 +308,7 @@ describe('Mailgun Configuration', () => {
   describe('isDevelopmentMode', () => {
     test('should return true when not in production and Mailgun not configured', () => {
       mockIsProduction.mockReturnValue(false);
+      mockGetEnvVar.mockReturnValue(undefined);
 
       expect(isDevelopmentMode()).toBe(true);
     });
@@ -347,6 +348,8 @@ describe('Mailgun Configuration', () => {
 
   describe('integration scenarios', () => {
     test('should work with real environment configuration flow', () => {
+      // Clear any previous mock implementations
+      mockGetEnvVar.mockReset();
       // Provide stable implementation for multiple calls
       mockGetEnvVar.mockImplementation((key: string, defaultValue?: string) => {
         switch (key) {