Fix Razor Pages CRUD model/DbContext discovery across referenced projects

[Copilot]

dotnet-scaffold 20.0.0 failed Razor Pages CRUD validation when --model and/or --dataContext types lived in referenced class libraries, causing Invalid --model and incorrect context handling. This change updates type discovery to include referenced-project symbols, aligning behavior with multi-project solution layouts.

• Razor Pages validation now resolves types from compilation symbols, not only source syntax trees

  • Updated model/DbContext lookup paths used by ValidateRazorPagesStep so public types in referenced assemblies are discoverable.
  • Preserved existing name matching behavior (including case-insensitive/simple-name resolution) while expanding symbol source.

• Roslyn code service symbol discovery expanded for cross-project scenarios

  • Adjusted class-symbol enumeration in CodeService to include reachable metadata/referenced type symbols in addition to in-project declarations.
  • Ensures scaffold input validation can resolve entity and context types regardless of which referenced project defines them.

• Focused coverage for referenced-project type resolution

  • Added/updated targeted tests to exercise model and DbContext discovery when those types are defined outside the web project.

Example of the affected resolution path:

// Before: only symbols from current project's syntax trees
var allClasses = await projectInfo.CodeService.GetAllClassSymbolsAsync();
var modelClassSymbol = allClasses.FirstOrDefault(
    x => x.Name.Equals(settings.Model, StringComparison.OrdinalIgnoreCase));

With this PR, GetAllClassSymbolsAsync() includes referenced-project type symbols, so Product and DiagnosisDbContext resolve correctly in the reproduced solution structure.

• Fixes [dotnet-scaffold 20.0.0] Invalid --model when model and DbContext are in referenced projects #3772