[io] Add type mismatch check for TObject-derived cases

[vepadulano]

Adds a check for type mismatch in TFileMerger for the particular case of having a TObject and trying to merge it against other objects of different types. Importantly, this includes also the check when the other object is of a non-TObject-derived type, which would previously lead to a segfault.

Fixes #21863

[github-actions]

Test Results

    22 files      22 suites   3d 3h 21m 5s ⏱️
 3 830 tests  3 829 ✅  1 💤 0 ❌
75 611 runs  75 593 ✅ 18 💤 0 ❌

Results for commit 1f70bda.

♻️ This comment has been updated with latest results.

[pcanal]

LGTM. Thanks.

[vepadulano]

Unrelated MacOS failure

2026-04-09T13:27:07.4979420Z 1555/3704 Test  #655: tutorial-analysis-dataframe-df027_SQliteDependencyOverVersion .....................................***Failed   10.02 sec
2026-04-09T13:27:07.4983300Z Processing /Users/sftnight/ROOT-CI/src/tutorials/analysis/dataframe/df027_SQliteDependencyOverVersion.C...
2026-04-09T13:27:07.5049100Z Minimum time is '2018-07-06 07:54:32'
2026-04-09T13:27:07.5075330Z libc++abi: terminating due to uncaught exception of type ROOT::RException: cannot read from http://root.cern/files/root_download_stats.sqlite: end of response with 85524 bytes missing [Transferred a partial file (18)] (OS errno: 0)
2026-04-09T13:27:07.5101820Z At:

Unrelated Windows failure

2026-04-09T13:17:05.4357117Z C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(254,5): error MSB8066: Custom build for 'C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-mkdir.rule;C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-download.rule;C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-update.rule;C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-patch.rule;C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-configure.rule;C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-build.rule;C:\ROOT-CI\build\CMakeFiles\11e3aa246d8d8a4f1578655b4fcb5e27\BUILTIN_LIBGIF-install.rule;C:\ROOT-CI\build\CMakeFiles\f42ef161ab91f48d11962adbc1640078\BUILTIN_LIBGIF-complete.rule;C:\ROOT-CI\build\CMakeFiles\16a4160b083f08e7c502201d14c6a681\BUILTIN_LIBGIF.rule;C:\ROOT-CI\src\builtins\libgif\CMakeLists.txt' exited with code 1. [C:\ROOT-CI\build\builtins\libgif\BUILTIN_LIBGIF.vcxproj]