Gemini API Cookbook: Fix malformed JSON in Get_started_managed_agents.ipynb (#1300)

Summary

Fix malformed JSON in Get_started_managed_agents.ipynb (#1300) * Fix malformed JSON in Get_started_managed_agents.ipynb Add three missing commas between adjacent string elements in cell source arrays. The notebook failed to parse as JSON (and therefore failed to open in Google Colab) with: SyntaxError: Expected ',' or ']' after array element in JSON at position 51200 (line 1193 column 15) There were three such spots (chars 51200, 66796, 67304), each an array string element missing its trailing comma. Adding the commas makes the notebook valid JSON again with no other content changes. Fixes…