Auto: opcuaclient-6 — Discovery URL FindServers

Adds optional `DiscoveryUrl` knob to OpcUaClientDriverOptions. When set,
the driver runs `DiscoveryClient.CreateAsync` + `FindServersAsync` +
`GetEndpointsAsync` against that URL during InitializeAsync and prepends
the discovered endpoint URLs (filtered to matching SecurityPolicy +
SecurityMode) to the failover candidate list. De-duplicates URLs that
appear in both discovered and static lists (case-insensitive). Discovery
failures are non-fatal — falls back to statically configured candidates.

The doc comment notes that FindServers requires SecurityMode=None on the
discovery channel per OPC UA spec, even when the data channel uses Sign
or SignAndEncrypt.

Closes #278

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverOptions.cs

@@ -39,6 +39,34 @@ public sealed class OpcUaClientDriverOptions
     /// </summary>
     public TimeSpan PerEndpointConnectTimeout { get; init; } = TimeSpan.FromSeconds(3);
 
+    /// <summary>
+    ///     Optional discovery URL pointing at a Local Discovery Server (LDS) or a server's
+    ///     own discovery endpoint. When set, the driver runs <c>FindServers</c> +
+    ///     <c>GetEndpoints</c> against this URL during <see cref="OpcUaClientDriver.InitializeAsync"/>
+    ///     and prepends the discovered endpoint URLs to the failover candidate list. When
+    ///     <see cref="EndpointUrls"/> is empty (and only <see cref="EndpointUrl"/> is set as
+    ///     a fallback), the discovered URLs replace the candidate list entirely so a
+    ///     discovery-driven deployment can be configured without specifying any endpoints
+    ///     up front. Discovery failures are non-fatal — the driver logs and falls back to the
+    ///     statically configured candidates.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         <b>FindServers requires SecurityMode=None on the discovery channel</b> per the
+    ///         OPC UA spec — discovery is unauthenticated even when the data channel uses
+    ///         <c>Sign</c> or <c>SignAndEncrypt</c>. The driver opens the discovery channel
+    ///         unsecured regardless of <see cref="SecurityMode"/>; only the resulting data
+    ///         session is bound to the configured policy.
+    ///     </para>
+    ///     <para>
+    ///         Endpoints returned by discovery are filtered to those matching
+    ///         <see cref="SecurityPolicy"/> + <see cref="SecurityMode"/> before being added to
+    ///         the candidate list, so a discovery sweep against a multi-policy server only
+    ///         surfaces endpoints the driver could actually connect to.
+    ///     </para>
+    /// </remarks>
+    public string? DiscoveryUrl { get; init; }
+
     /// <summary>
     ///     Security policy to require when selecting an endpoint. Either a
     ///     <see cref="OpcUaSecurityPolicy"/> enum constant or a free-form string (for