fix: correct MaxBytes enforcement and consumer start sequence after purge

StreamManager.Capture now accounts for full message size (subject +
payload + 16-byte overhead) when checking MaxBytes, matching Go's
memStoreMsgSize. PullConsumerEngine uses stream FirstSeq instead of
hardcoded 1 for DeliverAll after purge. Fix 6 tests with Go parity
assertions and updated MaxBytes values.

tests/NATS.Server.Tests/JetStream/JetStreamStreamEdgeCaseTests.cs

@@ -91,15 +91,17 @@ public class JetStreamStreamEdgeCaseTests
     [Fact]
     public async Task Discard_new_rejects_when_stream_at_max_bytes()
     {
+        // Message 1 = "discnew.a"(9) + payload(20) + overhead(16) = 45 bytes.
+        // MaxBytes=50 holds exactly one message; second is rejected.
         await using var fx = await JetStreamApiFixture.StartWithStreamConfigAsync(new StreamConfig
         {
             Name = "DISCNEW",
             Subjects = ["discnew.>"],
-            MaxBytes = 20,
+            MaxBytes = 50,
             Discard = DiscardPolicy.New,
         });
 
-        // Fill up the stream with small messages first
+        // Fill up the stream with one message
         var ack1 = await fx.PublishAndGetAckAsync("discnew.a", "12345678901234567890");
         ack1.ErrorCode.ShouldBeNull();
 
@@ -159,16 +161,18 @@ public class JetStreamStreamEdgeCaseTests
     [Fact]
     public async Task Max_msgs_with_discard_new_via_bytes_rejects_when_bytes_exceeded()
     {
-        // Use MaxBytes + DiscardNew to get the rejection path (pre-store check in Capture())
+        // Use MaxBytes + DiscardNew to get the rejection path (pre-store check in Capture()).
+        // Message = "maxnew.a"(8) + "1234567890"(10) + overhead(16) = 34 bytes.
+        // MaxBytes=40 holds exactly one message; second is rejected.
         await using var fx = await JetStreamApiFixture.StartWithStreamConfigAsync(new StreamConfig
         {
             Name = "MAXNEW",
             Subjects = ["maxnew.>"],
-            MaxBytes = 10,
+            MaxBytes = 40,
             Discard = DiscardPolicy.New,
         });
 
-        _ = await fx.PublishAndGetAckAsync("maxnew.a", "1234567890"); // 10 bytes, fills stream
+        _ = await fx.PublishAndGetAckAsync("maxnew.a", "1234567890"); // 34 bytes with overhead
 
         var rejected = await fx.PublishAndGetAckAsync("maxnew.c", "extra-data-overflows");
         rejected.ErrorCode.ShouldNotBeNull();