Fix asset browser "Open" action to use OS default app again

[Gopmyc]

Description

Restores the previous behavior of the Asset Browser "Open" contextual action.

After #686, "Open" for previewable assets (like textures/models) started going through the internal preview flow.
This change makes "Open" use the OS default application again, while keeping "Preview" unchanged for in-editor preview.

Scope is intentionally minimal:

• 1-line change in AssetBrowser.cpp
• no refactor
• no API changes
• no behavior change outside the "Open" contextual action

Related Issue(s)

Fixes #693

Review Guidance

Please focus on FileContextualMenu::CreateList() in Sources/OvEditor/src/OvEditor/Panels/AssetBrowser.cpp.

Changed behavior:

• "Open" now calls OvTools::Utils::SystemCalls::OpenFile(filePath.string())
• "Preview" still uses internal GUIHelpers::Open(...)

Expected result:

• Right-click texture/model -> "Open" launches default external software
• "Preview" still opens the in-editor preview flow

Screenshots/GIFs

N/A (behavioral fix, no UI layout change)

Checklist

• My code follows the project's code style guidelines
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes don't generate new warnings or errors

[adriengivry]

I had something else in mind for this one. I'll tackle it myself in the next dew days. Sorry!