feat: add gitlab duo-chat-gpt-6-sol and duo-chat-gpt-6-luna models - #7868

Adds GitLab Duo entries for OpenAI's GPT-6 Sol and GPT-6 Luna, following the existing duo-chat-gpt-6-astra.toml:

• providers/gitlab/models/duo-chat-gpt-6-sol.toml → base_model = "openai/gpt-6-sol"
• providers/gitlab/models/duo-chat-gpt-6-luna.toml → base_model = "openai/gpt-6-luna"

Both inherit context (1,050,000), output (128,000), temperature = false, and modalities (text/image/pdf in, text out) from the base models. reasoning_options matches providers/openai/models/gpt-6-{sol,luna}.toml. Cost is zeroed out because GitLab bills Duo proxy models upstream. Names follow the Agentic Chat (...) convention.

Source (ai-assist models.yml):

- name: "GPT-6 Sol"
  gitlab_identifier: "gpt_6_sol"
  max_context_tokens: 1050000
  proxy_provider: openai
  params:
    model: gpt-6-sol
    max_tokens: 128000
- name: "GPT-6 Luna"
  gitlab_identifier: "gpt_6_luna"
  max_context_tokens: 1050000
  proxy_provider: openai
  params:
    model: gpt-6-luna
    max_tokens: 128000

Provider mappings: https://gitlab.com/vglafirov/gitlab-ai-provider/-/merge_requests/47

Verified with bun validate: both resolve under gitlab.models with the same inherited limits and capabilities as duo-chat-gpt-6-astra.