refactor: moved registries into a separate module and code cleanup

fix: refactored dial provider to follow the same pattern

providers/registries/custom.py

@@ -0,0 +1,25 @@
+"""Registry loader for custom OpenAI-compatible endpoints."""
+
+from __future__ import annotations
+
+from ..shared import ModelCapabilities, ProviderType
+from .base import CAPABILITY_FIELD_NAMES, CapabilityModelRegistry
+
+
+class CustomEndpointModelRegistry(CapabilityModelRegistry):
+    """Capability registry backed by ``conf/custom_models.json``."""
+
+    def __init__(self, config_path: str | None = None) -> None:
+        super().__init__(
+            env_var_name="CUSTOM_MODELS_CONFIG_PATH",
+            default_filename="custom_models.json",
+            provider=ProviderType.CUSTOM,
+            friendly_prefix="Custom ({model})",
+            config_path=config_path,
+        )
+
+    def _finalise_entry(self, entry: dict) -> tuple[ModelCapabilities, dict]:
+        filtered = {k: v for k, v in entry.items() if k in CAPABILITY_FIELD_NAMES}
+        filtered.setdefault("provider", ProviderType.CUSTOM)
+        capability = ModelCapabilities(**filtered)
+        return capability, {}