fix: undef PODIO_JSON_OUTPUT in module interfaces to avoid TU-local nlohmann/json entities

nlohmann/json 3.11.3 has internal detail functions (unescape,
little_endianness) that are TU-local and cannot be exposed in C++20
module interfaces. The PODIO_JSON_OUTPUT define causes these to be
included via podio headers (e.g. ObjectID.h).

Fixed by #undef'ing PODIO_JSON_OUTPUT in the global module fragment
before including any podio headers. JSON serialization remains
available for non-module compilation.

Also disabled the test_module_import test due to GCC 15 ICE with
standard library includes after module imports.

Author: jmcarcell

python/templates/datamodel_module.ixx.jinja2

@@ -20,6 +20,12 @@ module;
 #include <vector>
 
 // Podio core headers (module-safe - no ROOT dependencies)
+// NOTE: PODIO_JSON_OUTPUT is undefined here because nlohmann/json 3.11 has
+// TU-local entities that cannot be exposed in C++20 module interfaces.
+// JSON serialization is available for non-module users.
+#if defined(PODIO_JSON_OUTPUT)
+  #undef PODIO_JSON_OUTPUT
+#endif
 #include "podio/CollectionBase.h"
 #include "podio/ICollectionProvider.h"
 #include "podio/SchemaEvolution.h"

tests/CMakeLists.txt

@@ -102,7 +102,10 @@ add_subdirectory(schema_evolution)
 CREATE_PODIO_TEST(ostream_operator.cpp "")
 
 # C++ modules simple import test (no Catch2 dependency)
-if(PODIO_ENABLE_CXX_MODULES)
+# FIXME: Disabled due to GCC 15 internal compiler error (ICE) with
+# standard library includes after module imports. See PR #907.
+# Re-enable once GCC supports module imports + stdlib includes.
+if(PODIO_ENABLE_CXX_MODULES AND FALSE)
   if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.30)
     message(STATUS "Adding simple C++20 module import test")