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/JetStreamStreamCrudTests.cs

@@ -29,11 +29,14 @@ public class JetStreamStreamCrudTests
     [Fact]
     public async Task Create_stream_with_discard_new_policy()
     {
+        // Each message size = subject + payload + 16 overhead.
+        // Two small messages: ("dn.one"=6 + "1"=1 + 16=23) + ("dn.two"=6 + "2"=1 + 16=23) = 46 bytes.
+        // MaxBytes=50 allows both small messages but rejects the large third one.
         await using var fx = await JetStreamApiFixture.StartWithStreamConfigAsync(new StreamConfig
         {
             Name = "DN",
             Subjects = ["dn.>"],
-            MaxBytes = 30,
+            MaxBytes = 50,
             Discard = DiscardPolicy.New,
         });
 
@@ -319,15 +322,20 @@ public class JetStreamStreamCrudTests
         state.Messages.ShouldBeLessThanOrEqualTo(3UL);
     }
 
-    // Go: TestJetStreamMaxBytesIgnored server/jetstream_test.go
+    // Go: TestJetStreamMaxBytesIgnored server/jetstream_test.go:15778
+    // Go test uses 10MB limit with 1MB messages, expects 9 messages retained.
+    // We use smaller sizes but same principle: MaxBytes caps stored bytes,
+    // accounting for per-message overhead (subject + 16 bytes).
     [Fact]
     public async Task Stream_with_max_bytes_discard_old_evicts_oldest()
     {
+        // Each stored message = subject("bytes.x"=7) + payload(20) + overhead(16) = 43 bytes.
+        // MaxBytes=200 holds ~4 messages (4*43=172). Publishing 20 should evict old ones.
         await using var fx = await JetStreamApiFixture.StartWithStreamConfigAsync(new StreamConfig
         {
             Name = "BYTES",
             Subjects = ["bytes.>"],
-            MaxBytes = 100,
+            MaxBytes = 200,
             Discard = DiscardPolicy.Old,
         });
 
@@ -335,7 +343,8 @@ public class JetStreamStreamCrudTests
             _ = await fx.PublishAndGetAckAsync("bytes.x", $"payload-{i:D10}");
 
         var state = await fx.GetStreamStateAsync("BYTES");
-        ((long)state.Bytes).ShouldBeLessThanOrEqualTo(100L);
+        ((long)state.Bytes).ShouldBeLessThanOrEqualTo(200L);
+        state.Messages.ShouldBeLessThan(20UL);
     }
 
     // Go: TestJetStreamMaxMsgsPerSubject server/jetstream_test.go