Steps to reproduce:
1. Create new Unity project
2. Open Package Manager window
3. Install Behaviour package (com.unity.behavior)
4. In Project window create Behaviour Graph (Create > Behaviour > Behavior Graph)
5. Observe the Console window

Actual results: Warnings are thrown in the Console after creating Behavior Graph
Expected results: No warnings thrown in the Console after creating Behavior Graph

Reproducible with versions: 1.0.13 (6000.4.0a1, 6000.4.0a4)
Not reproducible with versions: 1.0.13 (6000.3.0a6)

Tested on (OS): macOS Silicon Tahoe 26.0 (M1)

Warnings:

•  The type Unity.Behavior.Authoring Unity.Behavior.StartNodeModel is being serialized by [SerializeReference], but its parent type Unity.Behavior.GraphFramework Unity.Behavior.GraphFramework.BaseModel is missing the [Serializable] attribute. To suppress this warning add [Serializable] to Unity.Behavior.GraphFramework Unity.Behavior.GraphFramework.BaseModel or use [MakeSerializable].

• [Worker0] The type Unity.Behavior.Authoring Unity.Behavior.StartNodeModel is being serialized by [SerializeReference], but its parent type Unity.Behavior.GraphFramework Unity.Behavior.GraphFramework.BaseModel is missing the [Serializable] attribute. To suppress this warning add [Serializable] to Unity.Behavior.GraphFramework Unity.Behavior.GraphFramework.BaseModel or use [MakeSerializable].