Material migration sets shader field instead of unsetting it

[willeastcott]

Problem

When migrating legacy materials, the migration code sets data.shader to 'blinn' instead of unsetting it. This causes a validation error in the collab-server because the shader field is not defined in the material data schema.

Error

C3 Validation Error at: data.shader

The operation tries to set data.shader from "phong" to "blinn", but the schema doesn't recognize this field.

Root Cause

In src/editor/assets/assets-migrate.ts, the comment says "remove shader" but the code actually sets it:

// remove shader since physical material is now the default
if (asset.has('data.shader')) {
    const shader = asset.get('data.shader');
    asset.set('data.shader', 'blinn');  // BUG: should be unset
    ...
}

Compare this to how fresnelModel is correctly handled just below:

// remove fresnelModel since it is now always set to schlick
if (asset.has('data.fresnelModel')) {
    const fresnelModel = asset.get('data.fresnelModel');
    asset.unset('data.fresnelModel');  // Correct
    ...
}

Fix

Change asset.set('data.shader', 'blinn') to asset.unset('data.shader') to be consistent with how other deprecated fields are handled.

[willeastcott]

Fixed by #1711