feat(core): generate images and decisions through catalog models - #59420

Stacked on #59190, which defines the endpoints. This wires them up.

Problem

POST /api/experimental/generate/image and /decisions decode their requests and then always fail with 501. Image and decision models are in the catalog, and since #59380 each one points at a providers/<P>/images or /decisions entrypoint with the same model(modelID, settings) contract as chat, but nothing loads or runs them.

Change

POST /generate/image { model, prompt, images?, n?, aspect?, resolution? }
  Model.available(["image"]) → find the ref     not there: 404, another type: 422
  same settings path as chat                    provider + variant settings, stored key, ${VAR}s
  load providers/<P>/images → model(...)        not an image model: 422
  Image.generate                                429 retried, other provider errors: 502
  → { images (base64), tokens?, native? }

 model-resolver
+  loadCatalogPackage    settings + package loading, shared by every model type
   fromCatalogModel      chat: unchanged, now on top of loadCatalogPackage
+  fromCatalogMedia      image / decision: same package call, checks it got the right kind of model
+  resolveImage / resolveDecision(ref)
 generate
+  image, decisions      run the image and decision clients, map errors onto the endpoint's error table
 server                  501s replaced; question names still checked for duplicates first
 plugin                  ctx.generate.image / decisions (was pinned to text)

Smaller pieces:

• Image and decision models prefer the API whose entrypoint matches their type. 12 OpenRouter and fastrouter Gemini image models list chat-completions first and the images API second, and would otherwise resolve to the chat package.
• azure/images, google-vertex/images and the three Cloudflare entrypoints build their own URL, so they join BUILDS_OWN_URL and ignore the catalog's templated base_url.
• System One responses (OpenRouter, TypeSafe, Zen decisions) now fill usage.cost from the host's reported cost, so decision results return cost.
• cost is only what the provider reports. Image results have none yet, since per-image prices aren't in the catalog shape.
• Only rate limits are retried (3 times, backing off from 2s): an image or decision call that reached the provider may already be billed.

Out of scope

• Images through a chat API (50 image models only list chat-completions). They return 422 until there's an adapter for that.
• Session request hooks (Azure Entra ID tokens, ChatGPT sign-in) don't run here, the same as /generate/text today.

Verification

• bun run check passes; protocol and client check:generated pass.
• packages/core test/generate.test.ts: real catalog-shaped models through the real entrypoints with a fake HTTP client and a recorded OpenRouter System One response:
  • image: request URL, Bearer key from provider settings, model id; base64 result and token usage
  • decisions: choice, score and predicate answers in order, provider cost and tokens
  • a Google image variant's thinkingLevel reaches the request body, and a 400 maps to invalid_request
  • a chat model asked for images and an image model asked for decisions give UnsupportedModelError, an unknown ref gives ModelNotFoundError
• packages/core: test/model-resolver.test.ts, test/plugin, test/models.test.ts pass (532).
• packages/server test/generate.test.ts: unknown models give 404 on both routes, duplicate question names give 400.
• packages/ai, packages/plugin, packages/client: bun test passes.

Visual

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