fix: preserve custom properties when boolean modifier is applied during export

[Jefsky]

When "Apply Modifiers" is enabled and a Boolean modifier is active on an object, the to_mesh() call creates a new evaluated mesh datablock. If that evaluated mesh has any auto-generated properties (e.g. from the modifier evaluation pipeline), blender_data.keys() becomes non-empty, and the old code would skip copying custom properties from the original mesh — only removing blacklisted ones. This silently drops Python-registered custom properties on Mesh datablocks.

Root cause: The if len(blender_data.keys()) == 0 condition guarded the custom property copy. Any property present on the evaluated mesh (even irrelevant ones) flipped the behavior to delete-only.

Fix: Unconditionally copy non-blacklisted custom properties from the original mesh data to the evaluated mesh, using try/except to gracefully skip Blender 4.2+ statically-typed properties that cannot be overwritten. Blacklisted properties introduced by the modifier evaluation are still cleaned up.

How to test:

1. Open Blender, create a Cube
2. Run a script that registers a BoolProperty on bpy.types.Mesh (e.g. bpy.types.Mesh.testProp = bpy.props.BoolProperty(name="Test"))
3. Set the property to True
4. Add a Boolean modifier to the Cube and enable it in viewport
5. Export to glTF with "Apply Modifiers" and "Include Custom Properties" both checked
6. Verify the custom property appears in the glTF extras
7. Toggle the modifier off and re-export — property should still be present

Fixes #2700

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[julienduroure]

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