feat(bedrock): enable prompt caching toggle for custom ARN and default to ON

- Enable supportsPromptCache for custom ARN so the toggle appears
  (treating it as an advanced option where users know what they are doing)
- Default awsUsePromptCache to true instead of false for Bedrock provider

Fixes #10576

webview-ui/src/components/settings/providers/Bedrock.tsx

@@ -198,7 +198,7 @@ export const Bedrock = ({ apiConfiguration, setApiConfigurationField, selectedMo
 			{selectedModelInfo?.supportsPromptCache && (
 				<>
 					<Checkbox
-						checked={apiConfiguration?.awsUsePromptCache || false}
+						checked={apiConfiguration?.awsUsePromptCache ?? true}
 						onChange={handleInputChange("awsUsePromptCache", noTransform)}>
 						<div className="flex items-center gap-1">
 							<span>{t("settings:providers.enablePromptCaching")}</span>

webview-ui/src/components/ui/hooks/useSelectedModel.ts

@@ -211,11 +211,12 @@ function getSelectedModel({
 			const id = apiConfiguration.apiModelId ?? defaultModelId
 			const baseInfo = bedrockModels[id as keyof typeof bedrockModels]
 
-			// Special case for custom ARN.
+			// Special case for custom ARN - treat as advanced option where user knows what they're doing
+			// Enable prompt cache support since custom ARNs often point to newer Claude models
 			if (id === "custom-arn") {
 				return {
 					id,
-					info: { maxTokens: 5000, contextWindow: 128_000, supportsPromptCache: false, supportsImages: true },
+					info: { maxTokens: 5000, contextWindow: 128_000, supportsPromptCache: true, supportsImages: true },
 				}
 			}