feat: add GPT-5.5 and GPT-5.5 Pro to OpenAI chat models (#6278)

* feat(models): add gpt-5.5 and gpt-5.5-pro to chatOpenAI

  OpenAI released GPT-5.5 on April 23, 2026. Add both variants to the
  chatOpenAI model list with confirmed pricing:
  - gpt-5.5: /0 per 1M input/output tokens
  - gpt-5.5-pro: 0/80 per 1M input/output tokens

* feat(models): add gpt-5.5 and gpt-5.5-pro to chatOpenAI_LlamaIndex