feat: add OCSP peer reject and chain validation events (Gap 10.10)

Add OcspChainValidationEvent DTO, OcspStatus enum, and OcspEventBuilder
helper with BuildPeerReject, BuildChainValidation, and ParseStatus methods.
Register OcspChainValidationEvent in EventJsonContext source-gen context.
Add OcspPeerReject and OcspChainValidation subject constants to EventSubjects.
10 new tests in OcspEventTests cover all DTOs, builder methods, status
parsing, and JSON round-trip.

src/NATS.Server/Events/EventSubjects.cs

@@ -40,6 +40,11 @@ public static class EventSubjects
     // Inbox for responses
     public const string InboxResponse = "$SYS._INBOX_.{0}";
 
+    // OCSP advisory events
+    // Go reference: ocsp.go — OCSP peer reject and chain validation subjects.
+    public const string OcspPeerReject = "$SYS.SERVER.{0}.OCSP.PEER.REJECT";
+    public const string OcspChainValidation = "$SYS.SERVER.{0}.OCSP.CHAIN.VALIDATION";
+
     // JetStream advisory events
     // Go reference: jetstream_api.go advisory subjects
     public const string JsAdvisoryStreamCreated = "$JS.EVENT.ADVISORY.STREAM.CREATED.{0}";