feat(mail): Introduce EnvironmentProvider interface for SMTP settings

Introduced an  interface to abstract the retrieval of environment variables for the SMTP mail service.

Key changes:
- Created a new  interface with  methods.
- Refactored  to implement the new  interface.
- Updated  to depend on the  interface instead of the concrete  class.

This change allows for easier mocking of environment variables in tests, improving the overall testability of the mail service and promoting a more decoupled design.

Author: abhishekr700

api/src/main/java/com/google/appengine/api/mail/EnvironmentProvider.java

@@ -0,0 +1,22 @@
+package com.google.appengine.api.mail;
+
+/**
+ * An interface for providing environment variables.
+ */
+interface EnvironmentProvider {
+  /**
+   * Gets the value of the specified environment variable.
+   * @param name the name of the environment variable
+   * @return the string value of the variable, or {@code null} if the variable is not defined
+   */
+  String getenv(String name);
+
+  /**
+   * Gets the value of the specified environment variable, returning a default value if the
+   * variable is not defined.
+   * @param name the name of the environment variable
+   * @param defaultValue the default value to return
+   * @return the string value of the variable, or the default value if the variable is not defined
+   */
+  String getenv(String name, String defaultValue);
+}

api/src/main/java/com/google/appengine/api/mail/MailServiceImpl.java

@@ -53,15 +53,15 @@
  */
 class MailServiceImpl implements MailService {
   static final String PACKAGE = "mail";
-  private final SystemEnvironmentProvider envProvider;
+  private final EnvironmentProvider envProvider;
 
   /** Default constructor, used in production. */
   MailServiceImpl() {
     this(new SystemEnvironmentProvider());
   }
 
   /** Constructor for testing, allowing a mock environment provider. */
-  MailServiceImpl(SystemEnvironmentProvider envProvider) {
+  MailServiceImpl(EnvironmentProvider envProvider) {
     this.envProvider = envProvider;
   }
 

api/src/main/java/com/google/appengine/api/mail/SystemEnvironmentProvider.java

@@ -3,12 +3,13 @@
 /**
  * A simple wrapper around {@link System} to allow for easier testing.
  */
-class SystemEnvironmentProvider {
+class SystemEnvironmentProvider implements EnvironmentProvider {
   /**
    * Gets the value of the specified environment variable.
    * @param name the name of the environment variable
    * @return the string value of the variable, or {@code null} if the variable is not defined
    */
+  @Override
   public String getenv(String name) {
     return System.getenv(name);
   }
@@ -20,6 +21,7 @@ public String getenv(String name) {
    * @param defaultValue the default value to return
    * @return the string value of the variable, or the default value if the variable is not defined
    */
+  @Override
   public String getenv(String name, String defaultValue) {
     String value = System.getenv(name);
     return value != null ? value : defaultValue;