[Build] Expose processed fonts and assets for custom platform backends

[Redth]

Note

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It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description

The Resizetizer's ProcessMauiFonts and ProcessMauiAssets targets perform fundamentally platform-agnostic work (copying font files, resolving asset paths) but don't expose their results for consumption by custom platform backends.

This PR makes three small changes to Microsoft.Maui.Resizetizer.After.targets:

1. MauiProcessedFont public item group

After ProcessMauiFonts copies fonts to the intermediate directory, the copied files are exposed as MauiProcessedFont items. Custom backends can consume via AfterTargets="ProcessMauiFonts":

<Target Name="_MyPlatformBundleFonts" AfterTargets="ProcessMauiFonts">
    <ItemGroup>
        <BundleResource Include="@(MauiProcessedFont)">
            <LogicalName>%(Filename)%(Extension)</LogicalName>
        </BundleResource>
    </ItemGroup>
</Target>

2. MauiProcessedAsset public item group

After ProcessMauiAssets runs GetMauiAssetPath, the resolved assets are exposed as MauiProcessedAsset items.

3. Default _MauiAssetItemMetadata fallback

When no known platform matches (Android, iOS, Windows, Tizen), _MauiAssetItemMetadata now defaults to Link instead of being empty, which would cause GetMauiAssetPath to fail or produce no useful output.

Impact

• Zero breaking changes — existing platforms are unaffected
• ~11 lines of code total
• Fonts and assets "just work" for any backend that references the Resizetizer NuGet

Fixes #34221
Part of #34099

[Copilot]

Pull request overview

This PR updates the Resizetizer build targets to expose the results of font and asset processing so custom platform backends can consume the resolved/copied items without re-implementing that logic.

Changes:

• Adds a default fallback for _MauiAssetItemMetadata to Link when no known platform match is detected.
• Exposes resolved assets as a public @(MauiProcessedAsset) item group after GetMauiAssetPath runs.
• Exposes copied fonts as a public @(MauiProcessedFont) item group after the font copy step.

[Copilot]

MauiProcessedFont is populated from @(_MauiFontCopied), which is created via the wildcard $(_MauiIntermediateFonts)*. Since this directory also contains non-font outputs (e.g., MauiInfo.plist is generated into the same folder on iOS), incremental builds can leave extra files there and the wildcard will include them. That means custom backends consuming @(MauiProcessedFont) may end up bundling non-font files. Consider populating MauiProcessedFont from the Copy task's CopiedFiles output (or otherwise filtering @(_MauiFontCopied) to only .ttf/.otf etc.) so only actual font files are exposed.