feat(c++): make shared_ptr track ref by default

[chaokunyang]

Why?

Make C++ shared ownership fields track references by default and extend ref tracking semantics to SharedWeak to match intended ownership behavior and reduce required annotations.

What does this PR do?

• Make std::shared_ptr<T> (and fory::serialization::SharedWeak<T>) track refs by default in field metadata and serializers.
• Extend smart-pointer detection and option validation to include SharedWeak<T> alongside shared_ptr/unique_ptr.
• Update compile-time helpers, resolver logic, and tests to reflect the new default tracking behavior.
• Refresh C++ and xlang docs to document defaults and valid options for SharedWeak.

Related issues

#2906

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

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