fix(s7): a read pass that observes nothing reports nothing — empty pass no longer clears Faulted

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -672,10 +672,17 @@ public sealed class S7Driver
             // arms abandon the batch mid-pass, and a partially-observed pass must not stamp a
             // verdict. LastSuccessfulRead advances whenever ANYTHING read, even into a
             // Degraded/Faulted state — see ReadPassOutcome for why that is load-bearing.
-            _health = new DriverHealth(
-                outcome.WorstFailure,
-                outcome.AnySuccess ? now : _health.LastSuccessfulRead,
-                outcome.WorstFailure == DriverState.Healthy ? null : outcome.FirstError);
+            //
+            // A pass that observed NOTHING — an empty batch, or one whose every reference was
+            // unresolvable — reports nothing: it writes no health at all, preserving a prior
+            // Faulted (e.g. the access-denied config alert) that a Healthy write would have erased.
+            if (outcome.AnySuccess || outcome.WorstFailure != DriverState.Healthy)
+            {
+                _health = new DriverHealth(
+                    outcome.WorstFailure,
+                    outcome.AnySuccess ? now : _health.LastSuccessfulRead,
+                    outcome.WorstFailure == DriverState.Healthy ? null : outcome.FirstError);
+            }
         }
         finally { _gate.Release(); }
         return results;

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7.Tests/S7SpanReadParityTests.cs

@@ -555,6 +555,44 @@ public sealed class S7SpanReadParityTests
             before!.Value, "one tag read fine, so the watchdog's evidence must still advance");
     }
 
+    /// <summary>
+    ///     A pass that observes NOTHING — every reference unresolvable, so not one tag was read and
+    ///     not one failed — reports nothing: it must leave the driver's prior <c>Faulted</c> config
+    ///     alert (and its <c>LastSuccessfulRead</c>) exactly as it found them.
+    /// </summary>
+    /// <remarks>
+    ///     Deciding health once per pass means the pass has to answer "what did I observe?", and a
+    ///     degenerate pass observed nothing at all — writing <c>Healthy</c> for it would invent
+    ///     evidence and silently clear a PUT/GET config alert an operator still has to act on. The
+    ///     per-item health writes this replaced had that property for free (no item, no write); the
+    ///     end-of-pass guard is what preserves it.
+    /// </remarks>
+    [Fact]
+    public async Task A_pass_that_observes_nothing_leaves_a_prior_Faulted_untouched()
+    {
+        var ct = TestContext.Current.CancellationToken;
+        var factory = new ImageS7PlcFactory(Images());
+        factory.PerTagFaults["DB1.DBW0"] = new PlcException(ErrorCode.ReadData, "Accessing object not allowed");
+        using var driver = new S7Driver(Options(NarrowTags(), maxGap: -1), "s7-observes-nothing", factory);
+        await driver.InitializeAsync("{}", ct);
+
+        // Drive the driver into the access-denied config alert first.
+        _ = await driver.ReadAsync(["A"], ct);
+        var faulted = driver.GetHealth();
+        faulted.State.ShouldBe(DriverState.Faulted);
+
+        // Now a pass whose every reference is unresolvable: they are stamped BadNodeIdUnknown
+        // before the planner ever sees them, so no span, no multi-var item and no per-tag read
+        // happens — the pass observes neither a success nor a failure.
+        var results = await driver.ReadAsync(["NoSuchTag", "AlsoMissing"], ct);
+        results.ShouldAllBe(s => s.StatusCode == StatusBadNodeIdUnknown);
+
+        var after = driver.GetHealth();
+        after.State.ShouldBe(DriverState.Faulted, "an observe-nothing pass must not clear the config alert");
+        after.LastError.ShouldBe(faulted.LastError);
+        after.LastSuccessfulRead.ShouldBe(faulted.LastSuccessfulRead, "nothing was read, so nothing advances");
+    }
+
     /// <summary>
     ///     An answer of the wrong SHAPE — one item short — cannot be mapped back onto the request's
     ///     indexes, so the driver refuses to guess which tag got which value and falls the whole