Firebase cpp: Changed mutable_string() const to support returning value of static or small strings to fix non obvious behavior difference between this and the non const version.

Due to the wide spread use of mutable_string() to access string value regardless of string type, this function has changed to support returning all string types as std::string to not break existing code at runtime.

PiperOrigin-RevId: 275051789

Author: markchandler

app/src/include/firebase/variant.h

@@ -692,16 +692,16 @@ class Variant {
       return value_.small_string;
   }
 
-  /// @brief Const accessor for a Variant containing a mutable string only.
+  /// @brief Const accessor for a Variant containing a string.
   ///
   /// @note Unlike the non-const accessor, this accessor cannot "promote" a
-  /// static string to mutable, and thus will assert if the Variant you pass in
-  /// is not of MutableString type.
+  /// static string to mutable, and thus returns a std::string copy instead of a
+  /// const reference to a std::string
   ///
-  /// @return Reference to the string contained in this Variant.
-  const std::string& mutable_string() const {
-    assert_is_type(kTypeMutableString);
-    return *value_.mutable_string_value;
+  /// @return std::string with the string contents contained in this Variant.
+  std::string mutable_string() const {
+    assert_is_string();
+    return string_value();
   }
 
   /// @brief Const accessor for a Variant containing a vector of Variant data.