[watch] Static web asset abstraction

[tmat]

Add StaticWebAssetUpdateBuilder to HotReload.Client package so that we can reuse the logic of updating static web assets across all clients.

Fix a couple of issues:

• Align case sensitivity of string comparisons in StaticWebAsset helpers with ASP.NET tasks
• Avoid sending static web asset updates to running projects that are not web apps yet reference projects that have static web assets (e.g. Aspire host).

[Copilot]

Pull request overview

This PR refactors dotnet-watch static web asset hot reload to use a shared abstraction (StaticWebAssetUpdateBuilder) and centralizes static asset manifest filtering logic, with new unit tests covering scoped CSS and asset-path behaviors.

Changes:

• Introduces StaticWebAssetUpdateBuilder and moves static asset update graph-walking logic into a reusable component.
• Adds StaticWebAssetsManifest.GetFilesToWatch() and updates evaluation to use it (including filtering rules).
• Adds new unit tests for static web asset path helpers and multi-project update scenarios.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/Microsoft.DotNet.HotReload.Client.Tests/StaticWebAssetUpdateBuilderTests.cs	New tests covering update propagation across project graphs and scoped CSS bundle regeneration scenarios.
test/Microsoft.DotNet.HotReload.Client.Tests/StaticWebAssetTests.cs	New tests for StaticWebAsset helper methods (scoped CSS, bundles, compression, relative URL generation).
src/Dotnet.Watch/Watch/UI/IReporter.cs	Switches predefined test messages to use LogEvents entries for static web asset warnings.
src/Dotnet.Watch/Watch/HotReload/ProjectUpdatesBuilder.StaticWebAssets.cs	Extracts static asset update handling into a new partial file using StaticWebAssetUpdateBuilder.
src/Dotnet.Watch/Watch/HotReload/ProjectUpdatesBuilder.cs	Makes ProjectUpdatesBuilder partial and removes the inlined static asset update implementation.
src/Dotnet.Watch/Watch/Build/ProjectGraphUtilities.cs	Adjusts assembly name helper to extend ProjectInstance and adds ProjectInstanceId helpers.
src/Dotnet.Watch/Watch/Build/LoadedProjectGraph.cs	Adds a GetProjectNode(ProjectInstanceId) helper for retrieving graph nodes by project+TFM.
src/Dotnet.Watch/Watch/Build/EvaluationResult.cs	Uses StaticWebAssetsManifest.GetFilesToWatch() when adding watched static asset files.
src/Dotnet.Watch/HotReloadClient/Web/StaticWebAssetUpdateBuilder.cs	Adds the new abstraction for building per-application static asset updates + scoped CSS regeneration tracking.
src/Dotnet.Watch/HotReloadClient/Web/StaticWebAssetsManifest.cs	Adds GetFilesToWatch() to encapsulate filtering of manifest entries.
src/Dotnet.Watch/HotReloadClient/Web/StaticWebAsset.cs	Updates helper methods for case-insensitive extensions, adds OS-specific path comparison, and adds _content/ URL detection.
src/Dotnet.Watch/HotReloadClient/ProjectInstanceId.cs	Moves ProjectInstanceId into the HotReload namespace and enables nullable.
src/Dotnet.Watch/HotReloadClient/Logging/LogEvents.cs	Adds new log events for missing static web asset manifest and scoped CSS bundle file.