fix: update assignTask to use BufferAccumulator API

- Fix TypeScript error: assignTask() was still using string assignment
- Add extended timeout to session.test.ts afterAll hook to prevent
  flaky test failures during cleanup

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/session.ts

@@ -1317,9 +1317,9 @@ export class Session extends EventEmitter {
   assignTask(taskId: string): void {
     this._currentTaskId = taskId;
     this._status = 'busy';
-    this._terminalBuffer = '';
+    this._terminalBuffer.clear();
     this._outputBuffer = '';
-    this._textOutput = '';
+    this._textOutput.clear();
     this._errorBuffer = '';
     this._messages = [];
     this._lastActivityAt = Date.now();