Remove deprecated default_model constant

- Remove legacy default_model constant from PackageConstants
- Update llm_interface.py to raise error for unknown providers instead of falling back
- All provider defaults now consistently use default_models dict
- Improves error messages for unsupported providers

Author: Marius1311

src/cell_annotator/_constants.py

@@ -10,7 +10,6 @@ class PackageConstants:
     min_markers: int = 15
     use_raw: bool = False
     default_min_color_distance: float = 4.0  # Delta E threshold for color distinguishability
-    default_model: str = "gpt-4o-mini"  # Legacy default for backward compatibility
     default_models: dict[str, str] = {
         "openai": "gpt-4o-mini",
         "gemini": "gemini-2.5-flash-lite",

src/cell_annotator/model/llm_interface.py

@@ -61,7 +61,11 @@ def __init__(
             provider = self._detect_provider_from_model(model)
         elif provider is not None and model is None:
             # Provider specified, use default model for that provider
-            model = PackageConstants.default_models.get(provider, PackageConstants.default_model)
+            if provider not in PackageConstants.default_models:
+                raise ValueError(
+                    f"Unknown provider '{provider}'. Supported providers: {PackageConstants.supported_providers}"
+                )
+            model = PackageConstants.default_models[provider]
 
         # At this point, both provider and model should be strings
         assert provider is not None, "Provider should not be None at this point"