fix(#371): resolve TypeScript strictness errors in telemetry tracking

- llm-cost-table.ts: Add undefined guard for MODEL_COSTS lookup
- llm-telemetry-tracker.service.ts: Allow undefined in callingContext
  for exactOptionalPropertyTypes compatibility

Refs #371

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/llm/llm-cost-table.ts

@@ -80,7 +80,10 @@ export function getModelCost(modelName: string): ModelCost {
 
   for (const prefix of SORTED_PREFIXES) {
     if (normalized.startsWith(prefix)) {
-      return MODEL_COSTS[prefix];
+      const cost = MODEL_COSTS[prefix];
+      if (cost !== undefined) {
+        return cost;
+      }
     }
   }