[Docs] Document GPT-6 Astra provider support

[zoomote]

​Opened on behalf of @navedmerchant. View the task or mention @roomote for follow-up asks.

What changed

• Documents GPT-6 Astra's API and Codex availability, limits, reasoning levels, Responses-based tool requirements, caching, pricing, and temperature constraints.
• Adds the exact LiteLLM Responses-backed route and records verified OpenRouter, NanoGPT, and Vercel AI Gateway model IDs.
• Clarifies why general OpenAI-compatible tool calls, OpenCode Go, and Zoo Code's current Amazon Bedrock transport are not enabled for Astra.

Why this change was made

Provider availability changed during the Astra rollout. The documentation now matches the final verified implementation and gives users route-specific setup guidance without implying unsupported compatibility.

Impact

Users can identify which provider paths support Astra, which account plans are eligible for Codex access, and which request constraints Zoo Code applies. The default model remains unchanged.

Screenshots

Related PRs

• Implementation: [Feat] Add verified GPT-6 Astra support across providers Zoo-Code#1506
• Mutation CI follow-up: [Fix] Astra provider CI fails when focused tests are omitted Zoo-Code#1530