Unescape relative path before file lookup to fix space-in-filename issue

[jacob-l]

Summary

When serving files, if the requested path contains spaces or other special characters, the lookup fails because Uri.MakeRelativeUri returns a URL-encoded path (e.g. spaces become %20). The file provider expects a plain file path, not a URL-encoded one.

This PR adds a call to Uri.UnescapeDataString after MakeRelativeUri, ensuring the relative path is correctly unescaped before being used to retrieve files.

[dotnet-policy-service[bot]]

Thanks for your PR, @@jacob-l. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[ilonatommy]

@jacob-l thank you for your contribution. Could you, please add a test in https://github.com/dotnet/aspnetcore/blob/main/src/Components/WebView/WebView/test/StaticContentProviderTests.cs that would demonstrate the encoding failure you're describing?

[jacob-l]

@dotnet-policy-service agree

[jacob-l]

Thanks for your feedback and approval, @ilonatommy

[javiercn]

We also need security review, as unescaping has security implications.

[dotnet-policy-service[bot]]

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