feat(secrets): G-2a secret: arm on GalaxySecretRef via ISecretResolver (Task 7)

Add a secret:NAME arm to GalaxySecretRef.ResolveApiKey that resolves the Galaxy
gateway API key through the shared ISecretResolver — fail-closed if the secret is
absent (never falls through to the cleartext literal arm), retiring the dev:/literal
in-DB path for production. Because GetAsync is async the method becomes
ResolveApiKeyAsync; the await cascade threads ISecretResolver by ctor injection into
GalaxyDriver + GalaxyDriverBrowser and (since GalaxyDriver is built by a static
factory closure, not DI) through GalaxyDriverFactoryExtensions + DriverFactoryBootstrap
(which pulls the real resolver from the service provider — registered unconditionally
in Slice 1). A NullSecretResolver null-object backs the parse-only/test paths only;
the runtime path always gets the real resolver (verified end-to-end).

TDD: 3 new secret:-arm tests (resolve / fail-closed-on-absent / no-literal-warning)
RED without the arm, GREEN with it; 338 Galaxy tests pass; no sync-over-async.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/NullSecretResolver.cs

@@ -0,0 +1,26 @@
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
+
+/// <summary>
+///     A null-object <see cref="ISecretResolver"/> that resolves every name to <c>null</c>
+///     (absent). Used as the default for the internal test ctor and the parse-only
+///     <c>CreateInstance</c> path so callers that never exercise a <c>secret:</c> API-key
+///     ref need not thread a real resolver. A production <c>GalaxyDriver</c> always receives
+///     the DI-registered resolver via the factory; a <c>secret:</c> ref resolved against this
+///     null object throws fail-closed (the secret is reported absent), which is the correct
+///     behaviour for a mis-wired deployment.
+/// </summary>
+internal sealed class NullSecretResolver : ISecretResolver
+{
+    /// <summary>The shared singleton instance.</summary>
+    public static readonly NullSecretResolver Instance = new();
+
+    private NullSecretResolver()
+    {
+    }
+
+    /// <inheritdoc />
+    public Task<string?> GetAsync(SecretName name, CancellationToken ct) =>
+        Task.FromResult<string?>(null);
+}