Fix invalid MSBuild ItemGroup syntax in WASM HotReload

[lewing]

Fixes the invalid MSBuild syntax identified in PR #52879 review comments.

Issue

The original code in #52879 used incorrect MSBuild syntax:

<_WasmHotReloadModule OriginalItemSpec="%(Identity)" />

This creates a new empty item instead of updating the existing item's metadata, which could cause DefineStaticWebAssets to process phantom assets with empty Identity values.

Fix

Changed to the correct MSBuild Update pattern:

<_WasmHotReloadModule Update="@(_WasmHotReloadModule)">
  <OriginalItemSpec>%(Identity)</OriginalItemSpec>
</_WasmHotReloadModule>

This properly updates the existing item's metadata instead of creating a new item.

Review Feedback

This addresses the feedback from @copilot-pull-request-reviewer in #52879:
#52879 (comment)

<_WasmHotReloadModule OriginalItemSpec="%(Identity)" /> inside the ItemGroup appears to create a new item (with an empty Include) rather than setting metadata on the existing _WasmHotReloadModule item.

Testing

The existing test Publish_HostingMultipleBlazorWebApps_Works validates this scenario works correctly.

Risk

Very low - this is a surgical fix to use correct MSBuild syntax that follows the established pattern used elsewhere in the SDK.

[Copilot]

Pull request overview

Fixes an MSBuild ItemGroup declaration in the WASM SDK Hot Reload wiring so that OriginalItemSpec metadata is applied to the existing _WasmHotReloadModule item (rather than accidentally creating a new, empty item). This aligns with how DefineStaticWebAssets consumes OriginalItemSpec when PropertyOverrides is set.

Changes:

• Replace the invalid “create a new item with metadata” pattern with an MSBuild Update pattern.
• Ensure _WasmHotReloadModule has OriginalItemSpec metadata set to the item’s identity.