Fix Windows Release failures in DeepTreeEcho unit-test job

[Copilot]

The Unit Tests - DeepTreeEcho (windows-latest, Release) Actions job failed due to Windows-specific portability/linkage issues in test code and ReservoirCpp shared-library linkage. This PR makes targeted cross-platform and CMake fixes to unblock that job.

• Windows portability fixes in unit tests

  • Add missing <chrono> includes in test files that use std::chrono::high_resolution_clock.
  • Replace non-portable M_PI usage with a local constexpr PI constant in ReservoirIntegrationTests.cpp.

• Windows shared-library link fix (ReservoirCpp)

  • Enable WINDOWS_EXPORT_ALL_SYMBOLS for reservoircpp_core when building shared libraries on Windows.
  • This ensures the expected import library is produced/usable when linking deep_tree_echo.

• Files touched

  • DeepTreeEcho/Testing/UnitTests/ActiveInferenceTests.cpp
  • DeepTreeEcho/Testing/UnitTests/ReservoirIntegrationTests.cpp
  • ReservoirEcho/reservoircpp_cpp/CMakeLists.txt

Example of the portability adjustment:

#include <chrono>
constexpr double kPi = 3.14159265358979323846;
input(0) = std::sin(2.0 * kPi * t / 20.0);