Make EncodableValue::LongValue const (#25335)

This method doesn't mutate the value of the underlying variant.

Fixes https://github.com/flutter/flutter/issues/79472

shell/platform/common/client_wrapper/encodable_value_unittests.cc

@@ -33,10 +33,10 @@ TEST(EncodableValueTest, Int) {
 
 // Test the int/long convenience wrapper.
 TEST(EncodableValueTest, LongValue) {
-  EncodableValue value(std::numeric_limits<int32_t>::max());
-  EXPECT_EQ(value.LongValue(), std::numeric_limits<int32_t>::max());
-  value = std::numeric_limits<int64_t>::max();
-  EXPECT_EQ(value.LongValue(), std::numeric_limits<int64_t>::max());
+  const EncodableValue int_value(std::numeric_limits<int32_t>::max());
+  EXPECT_EQ(int_value.LongValue(), std::numeric_limits<int32_t>::max());
+  const EncodableValue long_value(std::numeric_limits<int64_t>::max());
+  EXPECT_EQ(long_value.LongValue(), std::numeric_limits<int64_t>::max());
 }
 
 TEST(EncodableValueTest, Long) {

shell/platform/common/client_wrapper/include/flutter/encodable_value.h

@@ -203,7 +203,7 @@ class EncodableValue : public internal::EncodableValueVariant {
   //
   // Calling this method if the value doesn't contain either an int32_t or an
   // int64_t will throw an exception.
-  int64_t LongValue() {
+  int64_t LongValue() const {
     if (std::holds_alternative<int32_t>(*this)) {
       return std::get<int32_t>(*this);
     }