Improve mail operator tests to correctly simulate variable substitution

The previous tests used ${address_list} as literal strings, which didn't
properly demonstrate how the parseList fix works. Updated tests now use
JSON string representations like ["a@ex.com", "b@ex.com"] which correctly
simulates what happens after Digdag's template engine processes variables.

This matches the pattern used in Config.verifyParseList() test and clearly
demonstrates that parseList can handle JSON strings while getList cannot.

Added detailed comments explaining the variable substitution process.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: i10416

digdag-standards/src/test/java/io/digdag/standards/operator/MailOperatorFactoryTest.java

@@ -357,27 +357,36 @@ public void errorCheckStringBcc()
     @Test
     public void sendToWithArrayVariable()
     {
+        // This test simulates what happens when you write:
+        //   _export:
+        //     address_list: ["bob@example.com", "charlie@example.com"]
+        //   +task:
+        //     mail>:
+        //     to: ${address_list}
+        //
+        // After template substitution, the "to" parameter becomes a JSON string like:
+        // '["bob@example.com", "charlie@example.com"]'
+        //
+        // The parseList method should parse this JSON string and convert it to a List
         Config config = mailConfig.deepCopy();
-        config.set("to", "${address_list}");
-        config.getNestedOrSetEmpty("_export")
-                .set("address_list", ImmutableList.of("bob@example.com", "charlie@example.com"));
+        config.set("to", "[\"bob@example.com\", \"charlie@example.com\"]");
 
         Operator op = factory.newOperator(newContext(
                 tempPath,
                 newTaskRequest().withConfig(config)));
         op.run();
+        greenMail.waitForIncomingEmail(5000, 2);
         receiveCheck("bob@example.com", 1);
         receiveCheck("charlie@example.com", 1);
     }
 
     @Test
     public void sendCcWithArrayVariable()
     {
+        // After template substitution, the "cc" parameter becomes a JSON string
         Config config = mailConfig.deepCopy();
         config.set("to", "bob@example.com");
-        config.set("cc", "${cc_list}");
-        config.getNestedOrSetEmpty("_export")
-                .set("cc_list", ImmutableList.of("charlie@example.com", "david@example.com"));
+        config.set("cc", "[\"charlie@example.com\", \"david@example.com\"]");
 
         Operator op = factory.newOperator(newContext(
                 tempPath,
@@ -392,11 +401,10 @@ public void sendCcWithArrayVariable()
     @Test
     public void sendBccWithArrayVariable()
     {
+        // After template substitution, the "bcc" parameter becomes a JSON string
         Config config = mailConfig.deepCopy();
         config.set("to", "bob@example.com");
-        config.set("bcc", "${bcc_list}");
-        config.getNestedOrSetEmpty("_export")
-                .set("bcc_list", ImmutableList.of("charlie@example.com", "david@example.com"));
+        config.set("bcc", "[\"charlie@example.com\", \"david@example.com\"]");
 
         Operator op = factory.newOperator(newContext(
                 tempPath,