* fix: use per-candidate provider for model_fallbacks
Each fallback model now uses its own api_base and api_key from
model_list instead of inheriting the primary model's provider config.
Previously, a single LLMProvider was created from the primary model's
ModelConfig and reused for all fallback candidates — only the model ID
string was swapped. This caused all fallback requests to be routed to
the primary provider's endpoint, making cross-provider fallback chains
non-functional (e.g., OpenRouter primary with Gemini fallback would
send the Gemini request to OpenRouter's API).
Fix: pre-create a per-candidate LLMProvider at agent initialization
time by looking up each candidate's ModelConfig from model_list. The
fallback run closure now selects the correct provider per candidate
via CandidateProviders map, falling back to agent.Provider when no
override is found.
Fixes#2140
Made-with: Cursor
test: add test for instance.go
fix: fix test
refactor: optimize
fix: fix Golang lint issues
chore: comment cleanup
* refactor: use resolvedModelConfig() instead of buildModelIndex()
* fix
6ce0306c66