Phase 3 PR 72 -- Multi-endpoint failover for OPC UA Client driver. Adds OpcUaClientDriverOptions.EndpointUrls ordered list + PerEndpointConnectTimeout knob. On InitializeAsync the driver walks the candidate list in order via ResolveEndpointCandidates and returns the session from the first endpoint that successfully connects. Captures per-URL failure reasons in a List<string> and, if every candidate fails, throws AggregateException whose message names every URL + its failure class (e.g. 'opc.tcp://primary:4840 -> TimeoutException: ...'). That's critical diag for field debugging -- without it 'failover picked the wrong one' surfaces as a mystery. Single-URL backwards compat: EndpointUrl field retained as a one-URL shortcut. When EndpointUrls is null or empty the driver falls through to a single-candidate list of [EndpointUrl], so every existing single-endpoint config keeps working without migration. When both are provided, EndpointUrls wins + EndpointUrl is ignored -- documented on the field xml-doc. Per-endpoint connect budget: PerEndpointConnectTimeout (default 3s) caps each attempt so a sweep over several dead servers can't blow the overall init budget. Applied via CancellationTokenSource.CreateLinkedTokenSource + CancelAfter inside OpenSessionOnEndpointAsync (the extracted single-endpoint connect helper) so the cap is independent of the outer Options.Timeout which governs steady-state ops. BuildUserIdentity extracted out of InitializeAsync so the failover loop builds the UserIdentity ONCE and reuses it across every endpoint attempt -- generating it N times would re-unlock the user cert's private key N times, wasteful + keeps the password in memory longer. HostName now reflects the endpoint that actually connected via _connectedEndpointUrl instead of always returning opts.EndpointUrl -- so the Admin /hosts dashboard shows which of the configured endpoints is currently serving traffic (primary vs backup). Falls back to the first candidate pre-connect so the dashboard has a sensible identity before the first connect, and resets to null on ShutdownAsync. Use case: an OPC UA hot-standby server pair (primary 4840 + backup 4841) where either can serve the same address space. Operator configures EndpointUrls=[primary, backup]; driver tries primary first, falls over to backup on primary failure with a clean AggregateException describing both attempts if both are down. Unit tests (OpcUaClientFailoverTests, 5 facts): ResolveEndpointCandidates_prefers_EndpointUrls_when_provided (list trumps single), ResolveEndpointCandidates_falls_back_to_single_EndpointUrl_when_list_empty (legacy config compat), ResolveEndpointCandidates_empty_list_treated_as_fallback (explicit empty list also falls back -- otherwise we'd produce a zero-candidate sweep that throws with nothing tried), HostName_uses_first_candidate_before_connect (dashboard rendering pre-connect), Initialize_against_all_unreachable_endpoints_throws_AggregateException_listing_each (three loopback dead ports, asserts each URL appears in the aggregate message + driver flips to Faulted). 31/31 OpcUaClient.Tests pass. dotnet build clean. OPC UA Client driver security/auth/availability feature set now complete per driver-specs.md \u00A78: policy-filtered endpoint selection (PR 70), Anonymous+Username+Certificate auth (PR 71), multi-endpoint failover (this PR).

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

@@ -13,9 +13,32 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
 /// </remarks>
 public sealed class OpcUaClientDriverOptions
 {
-    /// <summary>Remote OPC UA endpoint URL, e.g. <c>opc.tcp://plc.internal:4840</c>.</summary>
+    /// <summary>
+    ///     Remote OPC UA endpoint URL, e.g. <c>opc.tcp://plc.internal:4840</c>. Convenience
+    ///     shortcut for a single-endpoint deployment — equivalent to setting
+    ///     <see cref="EndpointUrls"/> to a list with this one URL. When both are provided,
+    ///     the list wins and <see cref="EndpointUrl"/> is ignored.
+    /// </summary>
     public string EndpointUrl { get; init; } = "opc.tcp://localhost:4840";
 
+    /// <summary>
+    ///     Ordered list of candidate endpoint URLs for failover. The driver tries each in
+    ///     order at <see cref="OpcUaClientDriver.InitializeAsync"/> and on session drop;
+    ///     the first URL that successfully connects wins. Typical use-case: an OPC UA server
+    ///     pair running in hot-standby (primary 4840 + backup 4841) where either can serve
+    ///     the same address space. Leave unset (or empty) to use <see cref="EndpointUrl"/>
+    ///     as a single-URL shortcut.
+    /// </summary>
+    public IReadOnlyList<string> EndpointUrls { get; init; } = [];
+
+    /// <summary>
+    ///     Per-endpoint connect-attempt timeout during the failover sweep. Short enough that
+    ///     cycling through several dead servers doesn't blow the overall init budget, long
+    ///     enough to tolerate a slow TLS handshake on a healthy server. Applied independently
+    ///     of <see cref="Timeout"/> which governs steady-state operations.
+    /// </summary>
+    public TimeSpan PerEndpointConnectTimeout { get; init; } = TimeSpan.FromSeconds(3);
+
     /// <summary>
     ///     Security policy to require when selecting an endpoint. Either a
     ///     <see cref="OpcUaSecurityPolicy"/> enum constant or a free-form string (for