refactor(scripts): scoped parent query + parent picker for multi-parent templates

Two caveats from the script-scope rollout addressed:

1. ITemplateEngineRepository.GetTemplatesComposingAsync — a scoped
   query that returns only the templates referencing a given template
   via Compositions, eager-loaded with their Attributes / Scripts /
   Compositions. Replaces the GetAllTemplatesAsync + filter pattern
   in TemplateEdit so the Monaco metadata fetch doesn't pull the
   entire template catalog to find one parent.

2. Multi-parent picker. The previous implementation suppressed Parent
   assistance entirely when more than one template composes the open
   one. Now TemplateEdit collects every parent into _editorParents
   and renders a small `select` above the script editor when there
   are >1, letting the user choose which parent's metadata drives
   Parent.Attributes / Parent.CallScript completion + diagnostics.
   Single-parent templates skip the picker (no UI change). Zero
   parents (root template) hide the picker and surface no Parent
   assistance.

Browser-verified on the Sensor Module template (composed by both Pump
and Variable Speed Motor): picker shows both options, switching
updates the editor's parent metadata immediately via the existing
GetContext callback.

Test counts unchanged (159 / 199); the new repo method is exercised
end-to-end by the parent-picker browser path.

src/ScadaLink.ConfigurationDatabase/Repositories/TemplateEngineRepository.cs

@@ -50,6 +50,16 @@ public class TemplateEngineRepository : ITemplateEngineRepository
             .ToListAsync(cancellationToken);
     }
 
+    public async Task<IReadOnlyList<Template>> GetTemplatesComposingAsync(int composedTemplateId, CancellationToken cancellationToken = default)
+    {
+        return await _context.Templates
+            .Where(t => t.Compositions.Any(c => c.ComposedTemplateId == composedTemplateId))
+            .Include(t => t.Attributes)
+            .Include(t => t.Scripts)
+            .Include(t => t.Compositions)
+            .ToListAsync(cancellationToken);
+    }
+
     public async Task AddTemplateAsync(Template template, CancellationToken cancellationToken = default)
     {
         await _context.Templates.AddAsync(template, cancellationToken);