fix: OpenRouter models stripped of prefix, causing 404 (#116)

When config has provider=openrouter and model=openrouter/free,
resolve_model_provider() stripped the 'openrouter/' prefix because
prefix == config_provider. This sent 'free' to OpenRouter's API,
which returned 404 (model not found).

OpenRouter always needs the full provider/model path (e.g.
openrouter/free, anthropic/claude-sonnet-4.6). The prefix-stripping
logic is only correct for direct-API providers.

Fix: skip prefix stripping entirely when config_provider is 'openrouter'.
Return the full model_id with provider='openrouter'.

Added 7 unit tests for resolve_model_provider() covering:
- openrouter/free keeps full path (the bug)
- openrouter cross-provider models keep full path
- direct API providers still strip prefix correctly
- cross-provider routing to openrouter
- bare model names use config provider
- empty model returns defaults

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/config.py

@@ -391,6 +391,10 @@ def resolve_model_provider(model_id: str) -> tuple:
 
     if '/' in model_id:
         prefix, bare = model_id.split('/', 1)
+        # OpenRouter always needs the full provider/model path (e.g. openrouter/free,
+        # anthropic/claude-sonnet-4.6). Never strip the prefix for OpenRouter.
+        if config_provider == 'openrouter':
+            return model_id, 'openrouter', config_base_url
         # If prefix matches config provider exactly, strip it and use that provider directly.
         # e.g. config=anthropic, model=anthropic/claude-... → bare name to anthropic API
         if config_provider and prefix == config_provider: