feat(security): DashboardTags on API-key constraints; sessions inherit owner tags (SEC-25 groundwork)

Adds a dashboard event-visibility tag to ApiKeyConstraints, riding in the existing
constraints JSON blob so no auth-store schema migration is needed (design
docs/plans/2026-07-10-dashboard-session-acl-tst15.md sections 3/3.1, open call
settled per its own recommendation). The tag is visibility-only: no read, write,
browse, or subscribe path consults it, and HasRead/HasWriteConstraints ignore it.

GatewaySession gains an immutable, ordinal-ignore-case Tags set stamped at
construction from the owning API key, forwarded by MxAccessGatewayService.OpenSession
from the resolved ApiKeyIdentity — never from the wire request, so a client cannot
label its own session with another tenant's tag. ISessionManager gains a tag-carrying
OpenSessionAsync overload whose default implementation forwards to the tagless one, so
an implementation that does not model tags opens an untagged (least visible) session.

apikey create-key gains --dashboard-tags team-a,team-b (repeatable, trimmed,
de-duplicated; an empty segment is rejected rather than dropped) and list-keys prints
the tags column. No enforcement yet — the EventsHub ACL that consumes the tag is a
later change.

src/ZB.MOM.WW.MxGateway.Server/Sessions/ISessionManager.cs

@@ -17,6 +17,33 @@ public interface ISessionManager
         string? ownerKeyId,
         CancellationToken cancellationToken);
 
+    /// <summary>
+    ///     Opens a new gateway session, stamping the owning API key's dashboard event-visibility
+    ///     tags onto it (SEC-25).
+    /// </summary>
+    /// <param name="request">Request payload.</param>
+    /// <param name="clientIdentity">Client identity string.</param>
+    /// <param name="ownerKeyId">API key identifier of the caller creating the session.</param>
+    /// <param name="ownerDashboardTags">
+    ///     The owner key's <c>ApiKeyConstraints.DashboardTags</c>. Null or empty opens an untagged
+    ///     session. Never sourced from the client's wire request — see
+    ///     <c>docs/plans/2026-07-10-dashboard-session-acl-tst15.md</c> §3.1.
+    /// </param>
+    /// <param name="cancellationToken">Token to cancel the asynchronous operation.</param>
+    /// <returns>The newly opened session.</returns>
+    /// <remarks>
+    ///     The default implementation forwards to the tagless overload, so an implementation that
+    ///     does not model tags (unit-test fakes) opens an <em>untagged</em> session. That is the
+    ///     fail-closed direction: untagged sessions are the least dashboard-visible ones.
+    /// </remarks>
+    Task<GatewaySession> OpenSessionAsync(
+        SessionOpenRequest request,
+        string? clientIdentity,
+        string? ownerKeyId,
+        IReadOnlyList<string>? ownerDashboardTags,
+        CancellationToken cancellationToken)
+        => OpenSessionAsync(request, clientIdentity, ownerKeyId, cancellationToken);
+
     /// <summary>Attempts to retrieve a session by ID.</summary>
     /// <param name="sessionId">Identifier of the session.</param>
     /// <param name="session">The retrieved session, if found.</param>