Annotate Java CopilotRequestContext.sessionId() as @nullable

sessionId() returns null when the request was issued outside any session,
but the nullability was only documented in the Javadoc. Every other SDK makes
this explicit in the type system (TypeScript string?, Python str | None,
.NET string?, Rust Option<String>). Add @nullable from the already-declared
spotbugs-annotations dependency on the field, constructor parameter, and getter
so SpotBugs and IDEs surface null-safety warnings, aligning Java with the other
SDKs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

java/src/main/java/com/github/copilot/CopilotRequestContext.java

@@ -8,6 +8,8 @@
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
 
+import edu.umd.cs.findbugs.annotations.Nullable;
+
 /**
  * The per-request context handed to every {@link CopilotRequestHandler} hook.
  * It exposes the routing and cancellation details of a single intercepted
@@ -18,6 +20,7 @@
 public final class CopilotRequestContext {
 
     private final String requestId;
+    @Nullable
     private final String sessionId;
     private final CopilotRequestTransport transport;
     private final String url;
@@ -26,7 +29,7 @@ public final class CopilotRequestContext {
 
     private LlmWebSocketResponseBridge webSocketResponse;
 
-    CopilotRequestContext(String requestId, String sessionId, CopilotRequestTransport transport, String url,
+    CopilotRequestContext(String requestId, @Nullable String sessionId, CopilotRequestTransport transport, String url,
             Map<String, List<String>> headers, CompletableFuture<Void> cancellation) {
         this.requestId = requestId;
         this.sessionId = sessionId;
@@ -53,6 +56,7 @@ public String requestId() {
      *
      * @return the session id, or {@code null}
      */
+    @Nullable
     public String sessionId() {
         return sessionId;
     }