mbproxy: close remaining 5 W3 test gaps from 2026-05-14 review

Closes the 5 "easily addable" W3 test gaps left after the prior W3 commit;
the 5 race-hard gaps remain documented as known omissions per the plan.
Tests: 382 pass / 0 fail (baseline 378 + 4 net new methods — the supervisor
runtime-fault test replaces the existing placeholder).

  #11 BcdCodecTests.Encode16_IntMinValue_Throws_OutOfRange_NoArithmeticSurprise
      Locks the (uint)value > Max16 boundary check against int.MinValue. The
      cast becomes 0x80000000 which is well above 9999, so the throw fires
      cleanly. Prevents regression to a two-sided int comparison that would
      underflow.

  #15 BcdPduPipelineTests.FC03_Request_QtyAbove128_AtNonBcdAddress_PassesThroughUnchanged
      DL205/DL260 caps FC03/FC04 at qty=128 (DL260/dl205.md). The proxy must
      NOT truncate the qty field — passing through unchanged lets the PLC's
      own validator return exception 03 to the client (transparent contract
      for FCs/addresses the rewriter doesn't own).

  #4 SupervisorTests.Supervisor_RuntimeFault_OnRunningListener_RecoversAndRebinds
      Replaces the previous placeholder. Genuinely faults the running listener
      mid-life by stopping its underlying TcpListener via reflection (the
      single externally-observable hook to force the accept loop's
      AcceptAsync to throw ObjectDisposedException). Asserts the supervisor
      transitions to Recovering, re-binds via the Polly pipeline, and bumps
      RecoveryAttempts.

  #10 HotReloadE2ETests.E2E_ReadCoalescingEnabled_FlipAtRuntime_PropagatesToOptionsMonitor
      Validates that flipping Mbproxy.Resilience.ReadCoalescing.Enabled at
      runtime via hot-reload propagates through the live IOptionsMonitor.
      The W2.1 fix wires the accessor through to add/restart supervisors;
      the multiplexer reads it per-PDU (unit-tested separately). Proving
      IOptionsMonitor sees the new value is sufficient for the contract.

  #16 ConfigReconcilerTests.Apply_ManyConcurrentReloads_With_PlcChurn_NoCorruption
      Stress-tests the W2.3 ConcurrentDictionary fix. 16 concurrent applies
      cycle through 8 distinct PLC rosters, driving Add+Remove churn against
      the live supervisor dict. Without W2.3 the inner Task.WhenAll
      continuations would corrupt Dictionary<,> and crash with
      KeyNotFoundException / ArgumentException. Asserts every apply succeeds,
      no orphan supervisors remain, and the reload counter equals 16.

The 5 deterministically-race-hard gaps (#5 TxId saturation propagation, #6
coalescing factory leak under saturation, #7 backend-reader head-of-line
block, #8 watchdog↔response race, #9 cascade↔new-accept race) remain open
by design — reproducing those races deterministically requires test seams in
production code or stress-style tests that flake on slow CI. The Wave-1
fixes are still verified at the unit-contract level
(UpstreamPipeTests.TrySendResponse_WhenChannelFull, etc.).

This closes everything actionable in codereviews/2026-05-14/RemediationPlan.md.

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

mbproxy/tests/Mbproxy.Tests/Proxy/BcdPduPipelineTests.cs

@@ -404,6 +404,27 @@ public sealed class BcdPduPipelineTests
         ctx.Counters.Snapshot().RewrittenSlots.ShouldBe(0);
     }
 
+    /// <summary>
+    /// Phase 12 (W3 test gap #15) — DL205/DL260 caps FC03/FC04 reads at qty=128 (above
+    /// Modbus spec's 125; documented in DL260/dl205.md). The proxy must NOT truncate the
+    /// qty field — a request with qty &gt; 128 at non-BCD addresses must pass through
+    /// unchanged so the PLC's own validator returns exception 03 to the client. This is
+    /// the transparent-pass-through contract for FCs and addresses the rewriter doesn't
+    /// own.
+    /// </summary>
+    [Fact]
+    public void FC03_Request_QtyAbove128_AtNonBcdAddress_PassesThroughUnchanged()
+    {
+        var ctx = MakeContext(BcdTag.Create(1024, 16)); // tag elsewhere; not in this read
+        var req = Fc03Request(address: 5000, qty: 200);
+        byte[] original = [..req];
+
+        Pipeline.Process(MbapDirection.RequestToBackend, ReadOnlySpan<byte>.Empty, req.AsSpan(), ctx);
+
+        req.ShouldBe(original, "qty must NOT be truncated; the PLC validates and returns ex03");
+        ctx.Counters.Snapshot().RewrittenSlots.ShouldBe(0);
+    }
+
     /// <summary>
     /// Phase 12 (W3 test gap) — symmetric inverse of the existing partial-overlap test:
     /// the write range starts ON the high register of a 32-bit pair (low word is BEFORE