[1/2] fix: document the name and description returned for each generation model

[dazzatronus]

GET /models and GET /models/{id} already return a name and description for each model, but GenerationModel doesn't declare them. The generated zod schema is strict, so the published package rejects every real response from these endpoints.

This declares both as optional strings, with examples taken from the live catalogue.

Verify: pnpm build and pnpm test pass on Node 22. Live /models, /models?expand=options and /models/{id} responses fail generationmodelGenerationModelSchema in 1.18.3 on name and description, and parse with this branch.

Stack

Order	PR	Adds	Why separate
1	#141 current	Declares name and description	Corrects the spec to the current response
2	#142	Adds available and unavailableReason	New field clients will rely on; revertible alone

Merge bottom-up: #141 into main, then retarget #142 to main.

flowchart LR
  A["Client lists generation models<br/>with its API key"] --> B{"Account's plan includes<br/>this kind of generation?"}
  B -- yes --> C["available: true"]
  B -- no --> D["available: false<br/>+ unavailableReason"]
  B -- "can't confirm" --> E["available omitted"]
  C --> F["Offer the model"]
  D --> G["Hide or disable the model"]
  E --> H["Treat as unknown;<br/>generation is still checked"]

Loading

Rules: available and unavailableReason, list endpoint guidance.

[github-actions]

🎉 This PR is included in version 1.18.4 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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