refactor: Remove unnecessary executor null checks in session creation

Since executor is now guaranteed to be non-null, removed redundant
null checks on lines 442 and 530 before calling session.setExecutor().

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

Author: brunoborges

src/main/java/com/github/copilot/sdk/CopilotClient.java

@@ -439,9 +439,7 @@ public CompletableFuture<CopilotSession> createSession(SessionConfig config) {
 
             long setupNanos = System.nanoTime();
             var session = new CopilotSession(sessionId, connection.rpc);
-            if (executor != null) {
-                session.setExecutor(executor);
-            }
+            session.setExecutor(executor);
             SessionRequestBuilder.configureSession(session, config);
             sessions.put(sessionId, session);
             LoggingHelpers.logTiming(LOG, Level.FINE,
@@ -527,9 +525,7 @@ public CompletableFuture<CopilotSession> resumeSession(String sessionId, ResumeS
             // Register the session before the RPC call to avoid missing early events.
             long setupNanos = System.nanoTime();
             var session = new CopilotSession(sessionId, connection.rpc);
-            if (executor != null) {
-                session.setExecutor(executor);
-            }
+            session.setExecutor(executor);
             SessionRequestBuilder.configureSession(session, config);
             sessions.put(sessionId, session);
             LoggingHelpers.logTiming(LOG, Level.FINE,