fix(ai-anthropic): populate cachedInputTokens in streaming responses (#6288)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Tim <hello@timsmart.co>

Author: 3 people

.changeset/anthropic-streaming-cached-input-tokens.md

@@ -0,0 +1,5 @@
+---
+"@effect/ai-anthropic": patch
+---
+
+populate `cachedInputTokens` in streaming responses from `cache_read_input_tokens`

packages/ai/anthropic/src/AnthropicLanguageModel.ts

@@ -971,6 +971,7 @@ const makeStreamResponse: (
           case "message_start": {
             // Track usage metadata
             usage.inputTokens = event.message.usage.input_tokens
+            usage.cachedInputTokens = event.message.usage.cache_read_input_tokens ?? undefined
             metaUsage = event.message.usage
 
             // Track response metadata