feat(provider): discover models for openai-compatible providers

[nikakoy131]

Adds automatic model discovery for OpenAI-compatible providers.

Changes

• Providers configured with a baseURL and no explicit models (using @ai-sdk/openai-compatible, explicitly or by default) are probed via GET <baseURL>/models during provider init. Discovered entries are mapped to model definitions (id, name, modalities from input_modalities, zero-cost defaults) and merged with any manually configured models — manual entries always win.
• Discovery failures (network error, non-200, malformed payload) are treated as "no models"; providers ending up with zero models are filtered out, and defaultModelIDs no longer throws for empty provider maps.
• When discovery returns nothing but the provider already has models (e.g. a catalog provider pointed at an unreachable endpoint), existing models are preserved.
• New Provider.refreshDiscovery() re-runs discovery; opencode models --refresh uses it to re-fetch discovered model lists.

Testing

• Unit tests for the discovery mapper (headers, auth, malformed payloads, modality mapping) plus provider-level tests (eligibility, manual override, refresh).
• Verified against a live llama.cpp server: a provider configured with only baseURL + apiKey lists its discovered models.
• Unreachable baseURL does not crash; the provider is simply omitted from listings.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found potential duplicate PRs:

1. feat(opencode): auto-discover models from OpenAI-compatible providers #32731 - feat(opencode): auto-discover models from OpenAI-compatible providers

   • feat(opencode): auto-discover models from OpenAI-compatible providers #32731
   • Directly addresses auto-discovery of models from OpenAI-compatible providers

2. feat(provider): add dynamic model discovery for custom providers #42660 - feat(provider): add dynamic model discovery for custom providers

   • feat(provider): add dynamic model discovery for custom providers #42660
   • Covers dynamic model discovery for providers with similar scope

3. feat(opencode): local LAN provider discovery + auto-discover models #27554 - feat(opencode): local LAN provider discovery + auto-discover models

   • feat(opencode): local LAN provider discovery + auto-discover models #27554
   • Includes auto-discovery of models as part of broader provider discovery feature

These PRs appear to be directly related or may already address the model discovery functionality for OpenAI-compatible providers. You should review their status and implementation to ensure PR #48486 isn't duplicating existing work.