Route tool ranker fallback through executor

src/ai/tool-ranker-fallback.ts

@@ -1,23 +1,56 @@
 import {ToolRankerFallbackPolicy} from "../common/policies.js";
+import {decidePipelineFallback, type PipelineFallbackDecision} from "./user-request-pipeline/fallback-executor.js";
 
 export type ToolRankerFallbackSelection = {
     toolNames: string[];
     usedRanker: boolean;
 };
 
+export type ToolRankerFallbackDecision = PipelineFallbackDecision & ToolRankerFallbackSelection;
+
+function fallbackActionForPolicy(policy: ToolRankerFallbackPolicy) {
+    return policy === ToolRankerFallbackPolicy.MAIN_MODEL
+        ? "use_alternate_target"
+        : "continue_without_stage";
+}
+
+export function decideToolRankerFallback(params: {
+    fallbackPolicy: ToolRankerFallbackPolicy;
+    availableToolNames: readonly string[];
+    reason: "unavailable" | "failed";
+}): ToolRankerFallbackDecision {
+    const action = fallbackActionForPolicy(params.fallbackPolicy);
+    const decision = decidePipelineFallback({
+        stage: "tool_rank",
+        reason: params.reason,
+        policies: [{
+            stage: "tool_rank",
+            onUnavailable: action,
+            onFailed: action,
+        }],
+    });
+
+    return {
+        ...decision,
+        toolNames: params.fallbackPolicy === ToolRankerFallbackPolicy.NO_TOOLS
+            ? []
+            : [...params.availableToolNames],
+        usedRanker: false,
+    };
+}
+
 export function resolveToolRankerFallbackSelection(params: {
     fallbackPolicy: ToolRankerFallbackPolicy;
     availableToolNames: readonly string[];
 }): ToolRankerFallbackSelection {
-    if (params.fallbackPolicy === ToolRankerFallbackPolicy.NO_TOOLS) {
-        return {
-            toolNames: [],
-            usedRanker: false,
-        };
-    }
+    const decision = decideToolRankerFallback({
+        fallbackPolicy: params.fallbackPolicy,
+        availableToolNames: params.availableToolNames,
+        reason: "failed",
+    });
 
     return {
-        toolNames: [...params.availableToolNames],
-        usedRanker: false,
+        toolNames: decision.toolNames,
+        usedRanker: decision.usedRanker,
     };
 }