Summary

• Fix MSVC LNK2005 linker errors when consumers build with LTCG (Link-Time Code Generation) enabled, reported in 1.15.0: linker errors with MSVC link-time-code-generation #5200
• Change #if defined(POCO_OS_FAMILY_WINDOWS) to #if defined(POCO_OS_FAMILY_WINDOWS) && defined(<Component>_EXPORTS) in all extern template declarations (headers) and explicit template instantiations (.cpp files) across Foundation, JSON, and Data/ODBC
• Enable LTCG (CMAKE_INTERPROCEDURAL_OPTIMIZATION=ON) on the windows-2025-msvc-cmake CI job to catch this class of bug going forward

Root cause

Commit daf19e5 stripped the API macro (e.g. Foundation_API) from extern template class declarations in headers on all Windows builds. This is correct when building the DLL (dllexport is invalid on extern template in MSVC), but wrong when consuming the DLL — the dllimport annotation is needed so the compiler knows the instantiation lives in the DLL. Without it, consumer TUs silently generate local instantiations, which cause LNK2005 duplicate symbol errors under LTCG.

The fix narrows the condition to only strip the macro when <Component>_EXPORTS is defined (i.e. when building the DLL itself).

Test plan

• macOS build passes (verified locally)
• Windows shared-library build with LTCG passes (CI windows-2025-msvc-cmake job, now with -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON)
• All existing test suites pass

Closes #5200