feat: add gitlab duo-chat-gpt-6-astra model

[vglafirov]

Adds providers/gitlab/models/duo-chat-gpt-6-astra.toml for GitLab Duo's OpenAI GPT-6 Astra offering.

• base_model = "openai/gpt-6-astra" — inherits context (1,050,000), output (128,000), modalities (text/image/pdf in, text out), and reasoning options from the existing providers/openai/models/gpt-6-astra.toml entry.
• Cost is zeroed out (input/output/cache_read/cache_write = 0) since GitLab Duo proxy models are billed upstream by GitLab, not directly by the underlying provider.
• name = "Agentic Chat (GPT-6 Astra)" follows the existing gitlab duo-chat-* naming convention (see duo-chat-gpt-5-6-sol.toml, duo-chat-opus-5.toml, etc.).

Source (ai-assist models.yml):

- name: "GPT-6 Astra"
  provider: "OpenAI"
  gitlab_identifier: "gpt_6_astra"
  max_context_tokens: 1050000
  proxy_provider: openai
  model_class_provider: "openai"
  params:
    model: gpt-6-astra
    max_tokens: 128_000

Verified with bun validate — the model resolves correctly under gitlab.models["duo-chat-gpt-6-astra"] with the inherited metadata from the base model.

[github-actions]

No actionable findings.