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.

src/NATS.Server/JetStream/Consumers/PullConsumerEngine.cs

@@ -263,7 +263,9 @@ public sealed class PullConsumerEngine
             DeliverPolicy.ByStartSequence when config.OptStartSeq > 0 => config.OptStartSeq,
             DeliverPolicy.ByStartTime when config.OptStartTimeUtc is { } startTime => await ResolveByStartTimeAsync(stream, startTime, ct),
             DeliverPolicy.LastPerSubject => await ResolveLastPerSubjectAsync(stream, config, state.LastSeq, ct),
-            _ => 1,
+            // Go: consumer.go — DeliverAll starts from stream's FirstSeq (not always 1).
+            // After purge, FirstSeq advances past deleted messages.
+            _ => state.FirstSeq > 0 ? state.FirstSeq : 1,
         };
     }