Update tour to use TogetherAI's API instead of OpenAI (#26466)

Due to OpenAI changing their free tier pricing and at least one user commenting that that broke the tour, update Part 3 of the tour to:

- use a simple fetch instead of openai sdk client
- use TOGETHER_API_KEY env var instead of OPENAI_API_KEY
- use one of Together's available models instead of GPT
- update text, filenames & code accordingly  (e.g. 3-gpt becomes 3-ai)

Tested the new convex-tour-chat-3 package in dev, seems to be working as expected

GitOrigin-RevId: cf6bb042f26dd8723ea1b44b7a1683366b0b82a8

Author: vakila