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/JetStreamStreamPolicyRuntimeTests.cs

@@ -7,11 +7,13 @@ public class JetStreamStreamPolicyRuntimeTests
     [Fact]
     public async Task Discard_new_rejects_publish_when_max_bytes_exceeded()
     {
+        // Each message = subject("s.a"=3) + payload(2) + overhead(16) = 21 bytes.
+        // MaxBytes=25 holds exactly one message; second publish is rejected.
         await using var fx = await JetStreamApiFixture.StartWithStreamConfigAsync(new StreamConfig
         {
             Name = "S",
             Subjects = ["s.*"],
-            MaxBytes = 2,
+            MaxBytes = 25,
             Discard = DiscardPolicy.New,
         });