Add Opper as a built-in OpenAI-compatible provider preset

[Felixkw12]

Using Opper with Zero today means setting up custom-openai-compatible and entering the endpoint, key variable and model by hand. I would like to propose a named opper preset, built the same way as the Atlas Cloud preset in #784.

Opper is an EU-hosted AI gateway: one OpenAI-compatible API and one key for 700+ models from 30+ providers, including Claude, GPT, Gemini, DeepSeek and Kimi. It is hosted in the EU (AWS Stockholm), and requests can be pinned to EU-hosted regions.

Proposed scope

One descriptor using the existing openAICompat helper:

openAICompat("opper", "Opper", "https://api.opper.ai/v3/compat", "claude-sonnet-4-6", []string{"OPPER_API_KEY"}),

• Bearer auth on the existing OpenAI-compatible chat, tool and streaming runtime. No new transport, dependency, OAuth flow or default-provider change.
• Placed in the aggregator cluster directly after openrouter, as asked in the review on feat(providers): add AI/ML API preset #402. Not badged as recommended and not above the first-party providers.
• No attribution, referral or partner headers. Requests go through plain CopyHeaders like every other preset.
• Curated picker entries in internal/providermodelcatalog/catalog.go: claude-sonnet-4-6 (default), gpt-5.5, gemini-3.8-flash, deepseek-v4-pro and kimi-k3. All five are pooled ids in Opper's public catalogue (GET https://api.opper.ai/v3/models), checked on 2026-10-02.
• Remote model list: ModelsDevProviderID falls through to the catalog id, and models.dev already lists provider opper, so remote.go needs no mapping line.

Files, mirroring #784:

• internal/providercatalog/catalog.go
• internal/providercatalog/catalog_test.go (expectedCatalogIDs, the TransportOpenAICompat order list, a TestOpperDescriptor)
• internal/providermodelcatalog/catalog.go
• internal/providermodelcatalog/catalog_test.go (a case in TestModelsAreProviderScoped)

No README or promotional changes.

If this is welcome, I will file the feature issue and open the PR once it has issue-approved.

Disclosure: I work at Opper.