Fix AppManifests processing

[dansiegel]

Cleanup note: I removed stale bot inline comments and rewrote Dan-authored review notes here to read naturally. The remaining ### 💡 Codex Review entry is a submitted bot review header, which is not editable/deletable by API, so it remains for traceability.

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[dansiegel]

I found two blockers.

• src/Mobile.BuildTools.XamarinSdk/Mobile.BuildTools.XamarinSdk.csproj: LangVersion=13.0 does not work with .NET 8 here (CSC : error CS1617: Invalid option '13.0' for /langversion when forcing 8.0.421). Use a version supported by the repo toolchain.
• src/Mobile.BuildTools.AppManifests/AppleManifests.targets: manifest generation targets _AppManifest / _PartialAppManifest, but current MAUI/.NET iOS targets consume $(AppBundleManifest) and @(PartialAppManifest) in _CompileAppManifest, so generated info-plist data can be missed.

This is important because the target-file text tests in this PR don’t evaluate the real Apple target graph, so this regression was not caught by those tests.

I reproduced the .NET 8 toolchain issue locally.

I’d still mark this as request-changes until resolved.

[dansiegel]

I found additional blockers:

• The packaged AppManifests target fails MSBuild evaluation as an SDK-style consumer. In a minimal net10.0-android restore/build, using the generated Mobile.BuildTools.AppManifests package raises:
  MSB4067: The element <Import> beneath element <When> is unrecognized.
  This is because Import can’t live under Choose/When and the package behavior never reaches the Android/iOS target flow. Import should be moved to top-level conditional Import entries.
• _MobileBuildToolsAppManifestTasksDll currently points at $(MSBuildThisFileDirectory)Mobile.BuildTools.dll, but the package ships Mobile.BuildTools.AppManifests.dll. After fixing import shape, task resolution will fail unless this is corrected.

Also missing here is at least one package-consumer smoke test that restores this nupkg into a real SDK-style target and runs target evaluation.

I also found LangVersion=13.0 in XamarinSdk with no observable need for C# 13, so this increases avoidable churn.

Given the above, I still need this as request-changes.

[dansiegel]

Latest commit updates now address the review items:

• global.json updated to the .NET 10 train (9.0.300/latestPatch) to avoid newer preview SDK selection in VS 2022 CI
• AppManifests imports converted to valid conditional <Import> elements
• task assembly path corrected to Mobile.BuildTools.AppManifests.dll
• missing task dependencies added to the standalone package
• Apple manifest wiring updated to set AppBundleManifest and PartialAppManifest while preserving legacy names
• LangVersion reduced to 12.0
• regression tests for target behavior added

I also re-ran restore/build/test and got green results, including packaged iOS smoke restore and PR checks.

[dansiegel]

I cleaned up a few stale review notes. The remaining Codex review entry is GitHub’s submitted review record, so I’m leaving it for traceability.

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