chore(core): refactor model resolution and cleanup fallback logic (#15228)

packages/core/src/availability/policyHelpers.ts

@@ -36,8 +36,6 @@ export function resolvePolicyChain(
   preferredModel?: string,
   wrapsAround: boolean = false,
 ): ModelPolicyChain {
-  // Availability uses the active/requested model directly. Legacy fallback logic
-  // (getEffectiveModel) only applies when availability is disabled.
   const modelFromConfig =
     preferredModel ?? config.getActiveModel?.() ?? config.getModel();