Fix build failures on Linux and Windows

[navidicted]

Description

This PR resolves build issues on both Linux and Windows when compiling the plugin with newer compilers.

Changes Included

1. Linux Build Fix

• Issue:
  • With GCC 15.2.1 compiler, the G_DEFINE_TYPE macro fails when the required class and instance types are not declared beforehand. This results in undefined type errors and prevents the plugin from building correctly, potentially leading to a MissingPluginException at runtime.
• Fix:
  • Added explicit forward typedef declarations for _AudioDecoderPlugin and _AudioDecoderPluginClass before the struct definitions so the macro can resolve the types correctly.

2. Windows Build Fix

• Issues:
  • MSVC defines legacy min and max macros via <windows.h>, which conflict with std::min and std::max, causing C2589 and C2059 compilation errors.
  • C4245 warning caused by an implicit conversion when passing MF_SOURCE_READER_MEDIASOURCE to a DWORD.
  • In newer Flutter Windows SDK version, flutter::EncodableMap and flutter::EncodableList types no longer transitively included via standart_method_codec.h and without fix it gives compilation error.
  • std::tolower returns int, assignment to char caused possible data loss warning.
• Fixes:
  • Wrapped std::min and std::max calls in parentheses (e.g. (std::min)(...)) to avoid macro expansion conflicts.
  • Added an explicit static_cast<DWORD> for MF_SOURCE_READER_MEDIASOURCE to match the expected type.
  • Added necessary library import into audio_decoder_plugin.h
  • Explicitly cast result to char, use unsigned char input to avoidd undefined behaviour for negative char values.

Environment Tested

• Linux: GCC 15.2.1
• Windows: MSVC toolchain