Avoid (Missing Reference) on embedded material asset fields

[Gopmyc]

Description

Asset fields were incorrectly showing "(Missing Reference)" for embedded material/texture references (virtual paths like model:embedded_material_*.ovmat), because existence checks used filesystem paths directly.

This PR updates the editor asset existence checker to:

• detect embedded asset paths,
• validate embedded material/texture references,
• resolve existence via the source model file,
• keep the previous behavior unchanged for non-embedded paths.

Related Issue(s)

Fixes #756

Review Guidance

• Import/load a model with embedded materials.
• In Inspector (Material Renderer) and material-related asset fields, verify embedded references no longer display "(Missing Reference)" when the source model exists.
• Verify regular invalid asset paths still display "(Missing Reference)".

Screenshots/GIFs

N/A

AI Usage Disclosure

Debugging

Checklist

• My code follows the project's code style guidelines
• When applicable, I have commented my code, particularly in hard-to-understand areas
• When applicable, I have updated the documentation accordingly
• My changes don't generate new warnings or errors
• I have reviewed and take responsibility for all code in this PR (including any AI-assisted contributions)