Investigate I/O of `std::unique_ptr` on latest MacOS beta

[vepadulano]

Check duplicate issues.

• Checked for duplicates

Description

As seen in #20354 , we don't yet fully understand why I/O of std::unique_ptr seems to misbehave on latest MacOS beta if the classes that it interacts with:

• Follow some dynamic polymorphism hierarchy
• Do not properly implement the destructors
• And do not include ClassDef macros (this last part especially seems to be a regression and contradicting what we say in our own docs)

Reproducer

Not available yet

ROOT version

Any

Installation method

Any

Operating system

MacOS Darwin Kernel Version 25.2.0

Additional context

No response

[hahnjo]

As discussed in #20354 (comment), the changes didn't help and likely also the conclusions drawn from it are wrong. This has high priority because it fails mac-beta on master, v6-38-00-patches, and v6-36-00-patches!