docs: add GPT-5.6 Sol and Terra to the predefined agent LLM models

[igorlukanin]

Adds GPT-5.6 Sol and Terra to the list of predefined models for an agent's llm property.

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[claude]

Claude finished @igorlukanin's task in 30s —— View job

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Review: docs-only addition — no issues found ✅

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What changed: two entries appended to the OpenAI GPT list of predefined llm values in docs-mintlify/admin/ai/index.mdx:93-94.

- `gpt_5_4`
+ `gpt_5_6_sol`
+ `gpt_5_6_terra`
- `o3`

Assessment:

• Correct file — this is the active /docs-mintlify site, not the deprecated /docs. ✅
• Placement and ordering are consistent with the surrounding list: version-ascending within the GPT group, o* reasoning models still last. ✅
• Naming style matches the existing convention (lowercase, _-separated version, variant suffix), same shape as gpt_5_mini. ✅
• Surgical edit to an existing page rather than new content — matches docs-mintlify/CLAUDE.md guidance. ✅
• No new page, so no docs.json navigation update is needed. ✅
• Nothing security-, performance-, or test-related applies to a two-line list addition. No test coverage gap: these identifiers aren't defined anywhere in this repo (grep for gpt_5_4/gpt_5_3 matches only this doc page), so the enum is validated on the Cube cloud side.

One thing I can't verify from this repo: because the model identifiers live outside the monorepo, I can't confirm that gpt_5_6_sol and gpt_5_6_terra are the exact strings the platform accepts. Worth a quick check against the cloud-side enum before merge — a typo here would only surface as a runtime config error for users.

Optional, not blocking: the list doesn't indicate which models are available on which plan tiers or which are recommended defaults, so readers picking a model get no guidance. That's pre-existing and out of scope for this PR.

• branch [`igor/cub-3581-document-gpt-5-6-models`](https://github.com/cube-js/cube/tree/igor/cub-3581-document-gpt-5-6-models)

[igorlukanin]

Checked the cloud-side enum: both keys match exactly. gpt_5_6_sol and gpt_5_6_terra are the strings added to PredefinedLLMProviders in cubedevinc/cube-runtime#1112 and to the predefined_llm enum in cubedevinc/cubejs-enterprise#13569, so no typo to catch. This PR lands last of the three for that reason.