Add OpenAPI specification for Link-Not-Merge Policy APIs

- Introduced a new OpenAPI YAML file for the StellaOps Concelier service.
- Defined endpoints for listing linksets, retrieving linksets by advisory ID, and searching linksets.
- Included detailed parameter specifications and response schemas for each endpoint.
- Established components for reusable parameters and schemas, enhancing API documentation clarity.

src/Concelier/__Libraries/StellaOps.Concelier.Core/Linksets/LinksetCorrelation.cs

@@ -110,7 +110,9 @@ internal static class LinksetCorrelation
             .Select(i => i.Purls
                 .Select(ExtractPackageKey)
                 .Where(k => !string.IsNullOrWhiteSpace(k))
+                .Select(k => k!)
                 .ToHashSet(StringComparer.Ordinal))
+            .Select(set => new HashSet<string>(set, StringComparer.Ordinal))
             .ToList();
 
         var seed = packageKeysPerInput.FirstOrDefault() ?? new HashSet<string>(StringComparer.Ordinal);