[Blazor] Treat RuntimeIdentifiers as local property

[maraf]

Extend "TreatAsLocalProperty" to RuntimeIdentifiers, the same way as we do for RuntimeIdentifier.
Projects using BlazorWebAssembly SDK are meant to target browser-wasm only.

Contributes to dotnet/runtime#110624

[github-actions]

This PR is targeting main, which is now for .NET 11-facing work. If you intended to target .NET 10, either retarget this PR to release/10.0.1xx or make sure you backport the change to release/10.0.1xx after merging. See #50394 for more details.

[dotnet-policy-service]

Thanks for your PR, @@maraf.
To learn about the PR process and branching schedule of this repo, please take a look at the SDK PR Guide.

[Copilot]

Pull Request Overview

This PR extends the BlazorWebAssembly SDK to treat RuntimeIdentifiers as a local property, similar to how RuntimeIdentifier is already handled. This ensures that Blazor WebAssembly projects consistently target only the browser-wasm runtime, preventing external settings from overriding this requirement.

Key changes:

• Add RuntimeIdentifiers to the TreatAsLocalProperty attribute
• Set RuntimeIdentifiers property to use the local RuntimeIdentifier value

[baronfel]

I'm unsure if we should just unset the property entirely or set it to $(RuntimeIdentifier) here. Did you try it both ways? Were there any behavioral differences?

[javiercn]

I would set this explicitly to browser-wasm

[maraf]

In my testing I didn't find a difference

[akoeplinger]

I have a vague recollection of some issue when both RuntimeIdentifiers and RuntimeIdentifier were set but I can't find any details anymore. Maybe it was just when they disagreed.

[baronfel]

It would be good to have a test validating the scenario from dotnet/runtime#110624.