fix(WRK-21): make drain budget monotonic at the reserve boundary; guard the reply-too-large fallback

Review follow-ups on the WRK-21 cluster.

1. ResolveDrainReplyByteBudget was a step function, not a floor: just above the
   64 KiB reserve the budget collapsed to a few bytes (at the validator-permitted
   floor MaxMessageBytes = 1024 + 64 KiB it was exactly 1024), too small to move a
   byte-heavy event, so DrainEvents truncated on every call and the drain-until-
   empty loop never terminated. It now takes the max of (frameMax - reserve) and
   frameMax/2, so the budget is monotonic and never below half the frame max. New
   test DrainEvents_AtValidatorFloorFrameMax_MakesProgressAndTerminates drives a
   byte-heavy queue at the exact validator floor and asserts it drains to empty
   with no head ever reported oversized.

2. The reply-too-large fallback write is now itself size-guarded
   (WriteReplyTooLargeFallbackAsync, used by both the control and STA reply seams):
   at a pathologically tiny negotiated max below the gateway's floor the fallback
   could also throw MessageTooLarge and — uncaught — kill the session, defeating the
   "no diagnostics command is session-fatal" invariant. It now log-and-swallows;
   comment notes WRK-24 adds the negotiated-max lower bound that makes it unreachable.

3. Corrected the RepeatedFieldOverheadBytes doc comments: WorkerEvent.CalculateSize()
   already includes the event's tag and length prefix (the same shape the reply's
   repeated events field packs), so the 8 bytes is pure slack over an already-
   conservative estimate, not compensation for a missing wrapper.

src/ZB.MOM.WW.MxGateway.Worker.Tests/Ipc/WorkerPipeSessionTests.cs

@@ -633,6 +633,76 @@ public sealed class WorkerPipeSessionTests
         await SendShutdownAndWaitAsync(pipePair, runTask, cancellation.Token);
     }
 
+    /// <summary>
+    ///     Regression for the reserve-boundary budget bug. The gateway validator accepts a Worker
+    ///     frame maximum as low as 1024 + 64 KiB, and just above that boundary a naive
+    ///     subtract-then-guard budget collapses to ~1024 bytes — too small to move even one
+    ///     byte-heavy event, so every drain reports truncation with the same head blocked and the
+    ///     drain-until-empty loop never terminates. The budget is now a floor (never below half the
+    ///     negotiated maximum), so a byte-heavy queue drains to empty even at the validator floor.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task DrainEvents_AtValidatorFloorFrameMax_MakesProgressAndTerminates()
+    {
+        // The lowest Worker.MaxMessageBytes GatewayOptionsValidator permits: the public gRPC floor
+        // (1024) plus the 64 KiB envelope-overhead reserve. The naive budget would be exactly 1024
+        // here; the floored budget is half of the frame max (~33 KiB).
+        const uint validatorFloorFrameMax = 1024 + (64 * 1024);
+        using CancellationTokenSource cancellation = new(TimeSpan.FromSeconds(60));
+        using PipePair pipePair = await PipePair.CreateAsync(cancellation.Token);
+        FakeRuntimeSession runtime = new()
+        {
+            SuppressDrainForBatchSize = 128,
+            BackingQueue = CreateByteHeavyQueue(200, ByteHeavyEventPayloadBytes),
+        };
+        WorkerPipeSession session = CreatePipeSession(pipePair.WorkerStream, runtime);
+        Task runTask = session.RunAsync(cancellation.Token);
+        await CompleteGatewayHandshakeAsync(pipePair, validatorFloorFrameMax, cancellation.Token);
+
+        int recovered = 0;
+        int replyCount = 0;
+        while (true)
+        {
+            string correlationId = $"floor-drain-{replyCount}";
+            await pipePair.GatewayWriter
+                .WriteAsync(
+                    CreateControlCommandEnvelope(
+                        correlationId,
+                        MxCommandKind.DrainEvents,
+                        command => command.DrainEvents = new DrainEventsCommand { MaxEvents = 0 }),
+                    cancellation.Token);
+
+            WorkerEnvelope replyEnvelope = await ReadUntilAsync(
+                pipePair.GatewayReader,
+                WorkerEnvelope.BodyOneofCase.WorkerCommandReply,
+                envelope => envelope.WorkerCommandReply.Reply.CorrelationId == correlationId,
+                cancellation.Token);
+            replyCount++;
+
+            MxCommandReply reply = replyEnvelope.WorkerCommandReply.Reply;
+            Assert.True(
+                replyEnvelope.CalculateSize() <= validatorFloorFrameMax,
+                $"reply {replyCount} serialized to {replyEnvelope.CalculateSize()} bytes.");
+
+            int drainedThisReply = reply.DrainEvents.Events.Count;
+            if (drainedThisReply == 0)
+            {
+                break;
+            }
+
+            // The head is never reported as oversized at this frame max: the ~33 KiB floored budget
+            // comfortably fits the ~1.8 KiB events, so each reply makes real progress.
+            Assert.DoesNotContain("alone exceeds", reply.DiagnosticMessage);
+            recovered += drainedThisReply;
+            Assert.True(replyCount < 200, "DrainEvents made no progress at the validator floor frame max.");
+        }
+
+        Assert.Equal(200, recovered);
+
+        await SendShutdownAndWaitAsync(pipePair, runTask, cancellation.Token);
+    }
+
     /// <summary>
     ///     Verifies the control-reply write seam is not session-fatal on size (WRK-21 backstop). The
     ///     reply builders size their payloads, so this path needs a deliberately budget-blind drain