refactor(opcuaclient): move NamespaceMap to Contracts, make public

Browser project (Phase 3) needs to share namespace-stable address encoding
with the runtime driver. Move keeps the same namespace, so existing usages
in OpcUaClientDriver compile unchanged.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/NamespaceMap.cs

@@ -32,7 +32,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
 ///         direct parse against the current session.
 ///     </para>
 /// </remarks>
-internal sealed class NamespaceMap
+public sealed class NamespaceMap
 {
     // index -> URI and URI -> index, as the upstream server published them at connect time.
     private readonly string[] _uris;

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts.csproj

@@ -5,5 +5,7 @@
         <ImplicitUsings>enable</ImplicitUsings>
         <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>
-    <!-- NO PackageReference. NO ProjectReference. -->
+    <ItemGroup>
+        <PackageReference Include="OPCFoundation.NetStandard.Opc.Ua.Client" />
+    </ItemGroup>
 </Project>