[6.x] Fix asset windows path issue

[jackmcdade]

Closes #6064

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Note

Low Risk
Small, targeted change to path normalization plus a regression test; minimal behavioral impact outside Windows-style ID inputs.

Overview
Fixes asset lookup by ID when the asset path contains Windows-style \\ separators.

AssetRepository::findById now normalizes the extracted path by converting backslashes to / before resolving the asset, and a new test asserts find('container::foo\\bar.jpg') correctly returns the saved asset.

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