Make configured OpenAI-compatible provider models directly selectable in AI-Chat #3696

Problem

A configured OpenAI-compatible provider (name, base URL, API key, model; ADR-095) is one endpoint plus the one model it targets. The AI-Chat selector listed whatever the endpoint's /models returned instead, which for an endpoint like OpenRouter is hundreds of rows, and nothing tied the list to what the user configured.

Expected Behavior

• The AI-Chat model selector and the Settings default-model dropdown list one row per configured provider (<name> · <model>, value openai-compat:<config-id>:<model>).
• Another model of the same endpoint is another provider entry.
• The provider form's Model field suggests the endpoint's discovered models; free text still works.

Acceptance Criteria

• Each provider with a configured model is one selectable option under "Remote endpoints".
• Selecting it sets provider: 'openai-compat' and modelId: 'openai-compat:<config-id>:<model>'.
• Discovered-but-unconfigured models are not listed in the selector.
• The Model field suggests the endpoint's discovered models.
• Tests cover one option per provider and the discovered-model name extraction.