refactor: break up cascade.py::complete() into helper methods (#1185)

Extract _get_providers_for_tier() and _try_single_provider() to reduce
the complexity of complete(). The method was 84 lines; now the main
logic is clearer and each helper has a single responsibility.

- _get_providers_for_tier(): Filters providers by cascade_tier
- _try_single_provider(): Attempts a single provider with metabolic protocol

Fixes #1185

src/infrastructure/router/cascade.py

@@ -528,6 +528,68 @@ class CascadeRouter:
 
         return True
 
+    def _get_providers_for_tier(self, cascade_tier: str | None) -> list[Provider]:
+        """Filter providers by tier, returning eligible providers."""
+        if cascade_tier == "frontier_required":
+            providers = [p for p in self.providers if p.type == "anthropic"]
+            if not providers:
+                raise RuntimeError("No Anthropic provider configured for 'frontier_required' tier.")
+            return providers
+        elif cascade_tier:
+            providers = [p for p in self.providers if p.tier == cascade_tier]
+            if not providers:
+                raise RuntimeError(f"No providers found for tier: {cascade_tier}")
+            return providers
+        return self.providers
+
+    async def _try_single_provider(
+        self,
+        provider: Provider,
+        messages: list[dict],
+        model: str | None,
+        temperature: float,
+        max_tokens: int | None,
+        content_type: ContentType,
+    ) -> dict | None:
+        """Attempt a single provider request.
+
+        Returns:
+            Response dict on success, None if provider should be skipped.
+        Raises:
+            RuntimeError: If the provider attempt fails.
+        """
+        if not self._is_provider_available(provider):
+            return None
+
+        # Metabolic protocol: skip cloud providers when quota is low
+        if provider.type in ("anthropic", "openai", "grok"):
+            if not self._quota_allows_cloud(provider):
+                logger.info(
+                    "Metabolic protocol: skipping cloud provider %s (quota too low)",
+                    provider.name,
+                )
+                return None
+
+        selected_model, is_fallback_model = self._select_model(provider, model, content_type)
+
+        result = await self._attempt_with_retry(
+            provider,
+            messages,
+            selected_model,
+            temperature,
+            max_tokens,
+            content_type,
+        )
+
+        self._record_success(provider, result.get("latency_ms", 0))
+        return {
+            "content": result["content"],
+            "provider": provider.name,
+            "model": result.get("model", selected_model or provider.get_default_model()),
+            "latency_ms": result.get("latency_ms", 0),
+            "is_fallback_model": is_fallback_model,
+        }
+
     async def complete(
         self,
         messages: list[dict],
@@ -561,55 +623,24 @@ class CascadeRouter:
         if content_type != ContentType.TEXT:
             logger.debug("Detected %s content, selecting appropriate model", content_type.value)
 
-        errors = []
-
-        providers = self.providers
-        if cascade_tier == "frontier_required":
-            providers = [p for p in self.providers if p.type == "anthropic"]
-            if not providers:
-                raise RuntimeError("No Anthropic provider configured for 'frontier_required' tier.")
-        elif cascade_tier:
-            providers = [p for p in self.providers if p.tier == cascade_tier]
-            if not providers:
-                raise RuntimeError(f"No providers found for tier: {cascade_tier}")
+        errors: list[str] = []
+        providers = self._get_providers_for_tier(cascade_tier)
 
         for provider in providers:
-            if not self._is_provider_available(provider):
-                continue
-
-            # Metabolic protocol: skip cloud providers when quota is low
-            if provider.type in ("anthropic", "openai", "grok"):
-                if not self._quota_allows_cloud(provider):
-                    logger.info(
-                        "Metabolic protocol: skipping cloud provider %s (quota too low)",
-                        provider.name,
-                    )
-                    continue
-
-            selected_model, is_fallback_model = self._select_model(provider, model, content_type)
-
             try:
-                result = await self._attempt_with_retry(
+                result = await self._try_single_provider(
                     provider,
                     messages,
-                    selected_model,
+                    model,
                     temperature,
                     max_tokens,
                     content_type,
                 )
+                if result is not None:
+                    return result
             except RuntimeError as exc:
                 errors.append(str(exc))
                 self._record_failure(provider)
-                continue
-
-            self._record_success(provider, result.get("latency_ms", 0))
-            return {
-                "content": result["content"],
-                "provider": provider.name,
-                "model": result.get("model", selected_model or provider.get_default_model()),
-                "latency_ms": result.get("latency_ms", 0),
-                "is_fallback_model": is_fallback_model,
-            }
 
         raise RuntimeError(f"All providers failed: {'; '.join(errors)}")