fix(anthropic): initialize reasoning field at zero

The Anthropic API does not expose a reasoning-token count even when
extended thinking is enabled — thinking cost is server-side folded into
output_tokens. Initialize usage.reasoning to 0 so the field is present
and add a regression guard so we do not later populate it from a
hallucinated payload field.

Author: yyyyaaa

packages/anthropic/__tests__/anthropic.test.ts

@@ -74,6 +74,7 @@ describe('AnthropicAdapter', () => {
     expect(message.stopReason).toBe('toolUse');
     expect(message.usage.input).toBe(12);
     expect(message.usage.output).toBe(4);
+    expect(message.usage.reasoning).toBe(0);
     expect(toolCall).toMatchObject({
       type: 'toolCall',
       id: 'tool_1',

packages/anthropic/src/index.ts

@@ -64,6 +64,7 @@ interface ToolCallContent {
 interface Usage {
   input: number;
   output: number;
+  reasoning: number;
   cacheRead: number;
   cacheWrite: number;
   totalTokens: number;
@@ -762,6 +763,7 @@ function createAssistantMessage(model: ModelDescriptor): AssistantMessage {
     usage: {
       input: 0,
       output: 0,
+      reasoning: 0,
       cacheRead: 0,
       cacheWrite: 0,
       totalTokens: 0,