Preview embedded model assets in asset browser

[Gopmyc]

Description

This PR makes model assets expandable in the Asset Browser to expose embedded materials and textures for quick preview.

Main changes:

• Models are now expandable in the Asset Browser.
• Embedded materials/textures are listed as read-only virtual entries.
• Embedded textures context menu now exposes only Preview.
• Embedded materials context menu now exposes:
  • Inspect (opens Material Editor)
  • Preview (opens Asset View)
• Double-click on embedded entries still opens Asset View.
• Open routing now recognizes embedded resource paths explicitly.

Related Issue(s)

Fixes #678

Review Guidance

Please focus review on:

• Asset Browser behavior for expanded model nodes
• Context menu behavior for embedded material/texture entries
• Open behavior differences:
  • embedded material Inspect -> Material Editor
  • embedded material Preview -> Asset View
  • embedded texture Preview -> Asset View

Screenshots/GIFs

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AI Usage Disclosure

Refactored code / 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)

[adriengivry]

Looks good! Thanks for putting that together!
A few comments:

• To be consistent with other contextual menu, embedded files should only have an option to "Open", which would do the same as non-embedded files:
  • for materials: Preview + Open in material
  • for textures: Preview
• Embedded assets (materials & textures) should be drag-and-droppable

[Gopmyc]

Looks good! Thanks for putting that together! A few comments:

• To be consistent with other contextual menu, embedded files should only have an option to "Open", which would do the same as non-embedded files:

  • for materials: Preview + Open in material
  • for textures: Preview

• Embedded assets (materials & textures) should be drag-and-droppable

Done ! I aligned embedded file menus to a single Open action and made embedded materials/textures drag-and-droppable