Move kcenon ecosystem packages to optional 'ecosystem' feature to allow
CI builds with standard vcpkg packages. This follows the same pattern
used in network_system and database_system.

Default dependencies now use standard vcpkg packages (asio, fmt,
nlohmann-json, spdlog) that are available in the official registry.

The ecosystem feature will be enabled once kcenon packages are
registered in the vcpkg registry (tracked in common_system#251).

Add --x-feature=testing flag to vcpkg install commands to install
gtest and benchmark dependencies that were moved to the testing
feature in vcpkg.json.

The gmock feature was removed in vcpkg's gtest port since it is now
included by default. Using gtest[gmock] causes build failure:
"gtest@1.17.0#2 does not have required feature gmock"

This fixes the Windows MSVC CI build failure.

Set VCPKG_MANIFEST_FEATURES before project() to ensure vcpkg installs
gtest and benchmark when tests are enabled. This fixes the CMake error:
"Could NOT find GTest (missing: GTEST_LIBRARY GTEST_INCLUDE_DIR)"

The vcpkg toolchain is loaded during project() call, so manifest
features must be configured beforehand.

Reverts the CMakeLists.txt changes from the previous commit.
Setting VCPKG_MANIFEST_FEATURES causes CMake to re-run vcpkg install
with a different installation path, breaking the build when vcpkg
packages were pre-installed by CI to a different directory.

The CI workflow already passes --x-feature=testing to vcpkg install,
so this configuration is not needed.

Pass VCPKG_INSTALLED_DIR to use pre-installed packages from CI,
and VCPKG_MANIFEST_FEATURES to enable the testing feature.

This ensures CMake uses the packages installed by the vcpkg install
step rather than re-running vcpkg with default features only.

Add 'testing' to vcpkg_installed cache key to ensure cache invalidation
when switching between builds with and without testing dependencies.

This fixes the GTest not found error on Windows where the cache was
created before testing feature was enabled in vcpkg.json.

- Remove VCPKG_INSTALLED_DIR and VCPKG_MANIFEST_FEATURES from Unix build
  (Unix uses system libraries via apt/brew)
- Set VCPKG_MANIFEST_MODE=OFF for Windows to use packages installed by
  vcpkg install step instead of attempting to reinstall
- This fixes the asio.hpp not found error caused by path mismatch
  between vcpkg install and vcpkg toolchain

Revert VCPKG_MANIFEST_MODE=OFF as it disabled vcpkg toolchain's
include path configuration. Use VCPKG_MANIFEST_FEATURES instead
to let vcpkg toolchain handle package installation in manifest mode.

Skip manual vcpkg install step on Windows and let vcpkg toolchain
install packages in manifest mode. This ensures packages are installed
in CMAKE_BINARY_DIR/vcpkg_installed which is the default path vcpkg
toolchain uses, avoiding path mismatch issues.

vcpkg binary caching is enabled so package installation remains fast.

- Re-enable Windows vcpkg install step (runs on cache miss)
- Add VCPKG_INSTALLED_DIR to point CMake to correct package location
- Set VCPKG_MANIFEST_MODE=OFF to use pre-installed packages
- This ensures vcpkg toolchain finds packages at the cached location

…E=OFF

VCPKG_MANIFEST_MODE=OFF disables vcpkg toolchain's include path setup.
VCPKG_MANIFEST_INSTALL=OFF keeps manifest mode active but skips auto-install,
allowing the toolchain to properly configure include paths for pre-installed packages.

- Remove manual vcpkg install step for Windows
- Let vcpkg toolchain handle installation via manifest mode
- Use VCPKG_MANIFEST_FEATURES=testing to enable testing dependencies
- Windows uses vcpkg binary caching (x-gha) for build speed
- Unix continues to use manual vcpkg install with directory caching

network_system links asio as PRIVATE, so asio headers are not propagated
to dependent targets. This workaround finds and links asio directly with
PUBLIC visibility to ensure asio.hpp is available when compiling
network_system headers included by messaging_system.