fix: correct task scheduling and simplify agent internals

Fix queue cancellation, concurrency limits and worker status updates.

Use direct Explorer imports, split the Diagnoser loop into helpers and remove unused tool registration and prompt-builder code.

Correct model caching, endpoint selection and retry handling.

artemis/llm/router.py

@@ -20,6 +20,7 @@ Anthropic, OpenRouter, and local Ollama/vLLM endpoints.
 """
 
 from enum import StrEnum
+import hashlib
 import os
 from typing import Any
 
@@ -46,10 +47,19 @@ class ModelProvider(StrEnum):
 
     @classmethod
     def from_string(cls, val: Any) -> "ModelProvider":
-        """Converts any string, enum, or mock representation into a canonical ModelProvider."""
+        """Converts a string or enum into a canonical ModelProvider.
+
+        ``None``/empty means "use the default" (Google); an unrecognized
+        non-empty value raises instead of silently routing to Gemini with
+        the wrong credentials.
+        """
         if isinstance(val, cls):
             return val
+        if val is None:
+            return cls.GOOGLE
         s = str(val).lower().replace("_", "").replace("-", "").strip()
+        if not s:
+            return cls.GOOGLE
         mapping = {
             "google": cls.GOOGLE,
             "gemini": cls.GOOGLE,
@@ -65,7 +75,12 @@ class ModelProvider(StrEnum):
             "vllm": cls.VLLM,
             "custom": cls.CUSTOM,
         }
-        return mapping.get(s, cls.GOOGLE)
+        provider = mapping.get(s)
+        if provider is None:
+            raise ValueError(
+                f"Unknown LLM provider {val!r}. Valid providers: {sorted(set(mapping))}"
+            )
+        return provider
 
 
 class ModelEndpoint(BaseModel):
@@ -97,11 +112,15 @@ class ModelEndpoint(BaseModel):
     )
 
     def cache_key(self) -> tuple:
-        """Returns a deterministic hashable cache key for connection pooling."""
+        """Key the client cache by endpoint settings, hashing the API key."""
+        api_key_digest = (
+            hashlib.sha256(self.api_key.encode("utf-8")).hexdigest()[:16] if self.api_key else None
+        )
         return (
             self.provider.value,
             self.model_name,
             self.temperature,
+            self.max_tokens,
             self.timeout_seconds,
             self.thinking_budget,
             self.thinking_level,
@@ -109,6 +128,7 @@ class ModelEndpoint(BaseModel):
             self.reasoning_effort,
             self.enable_grounding,
             self.api_base,
+            api_key_digest,
         )