Use const reference for loop variables.

* addresses the compiler warnings

```
composer/key_parser_test.cc:81:19: warning: loop variable '<structured bindings>' creates a copy from type 'const std::pair<std::basic_string_view<char>, unsigned int>' [-Wrange-loop-construct]
   81 |   for (const auto [name, modifiers] : kTestData) {
      |                   ^~~~~~~~~~~~~~~~~
composer/key_parser_test.cc:81:19: note: use reference type to prevent copying
   81 |   for (const auto [name, modifiers] : kTestData) {
      |                   ^~~~~~~~~~~~~~~~~
      |                   &
composer/key_parser_test.cc: In member function 'virtual void mozc::{anonymous}::KeyParserTest_SpecialKeys_Test::TestBody()':
composer/key_parser_test.cc:181:19: warning: loop variable '<structured bindings>' creates a copy from type 'const std::pair<std::basic_string_view<char>, mozc::commands::KeyEvent_SpecialKey>' [-Wrange-loop-construct]
  181 |   for (const auto [name, modifiers] : kTestData) {
      |                   ^~~~~~~~~~~~~~~~~
composer/key_parser_test.cc:181:19: note: use reference type to prevent copying
  181 |   for (const auto [name, modifiers] : kTestData) {
      |                   ^~~~~~~~~~~~~~~~~
      |                   &
```

#codehealth

PiperOrigin-RevId: 603603821

src/composer/key_parser_test.cc

@@ -78,7 +78,7 @@ TEST(KeyParserTest, ModifierKeys) {
       {"SHIFT", KeyEvent::SHIFT},
   };
 
-  for (const auto [name, modifiers] : kTestData) {
+  for (const auto& [name, modifiers] : kTestData) {
     SCOPED_TRACE(name);
     KeyEvent key_event;
     EXPECT_TRUE(KeyParser::ParseKey(name, &key_event));
@@ -178,7 +178,7 @@ TEST(KeyParserTest, SpecialKeys) {
       {"on", KeyEvent::ON},
   };
 
-  for (const auto [name, modifiers] : kTestData) {
+  for (const auto& [name, modifiers] : kTestData) {
     SCOPED_TRACE(name);
     KeyEvent key_event;
     EXPECT_TRUE(KeyParser::ParseKey(name, &key_event));