fix(usage): parse Kimi K2 cached_tokens from prompt_tokens_details

Kimi K2 models use automatic prefix caching and return cache stats in
a nested field: usage.prompt_tokens_details.cached_tokens

This fixes issue #7073 where cacheRead was showing 0 for K2.5 users.

Also adds cached_tokens (top-level) for moonshot-v1 explicit caching API.

Closes #7073

src/agents/usage.test.ts

@@ -54,6 +54,40 @@ describe("normalizeUsage", () => {
     });
   });
 
+  it("handles Moonshot/Kimi cached_tokens field", () => {
+    // Moonshot v1 returns cached_tokens instead of cache_read_input_tokens
+    const usage = normalizeUsage({
+      prompt_tokens: 30,
+      completion_tokens: 9,
+      total_tokens: 39,
+      cached_tokens: 19,
+    });
+    expect(usage).toEqual({
+      input: 30,
+      output: 9,
+      cacheRead: 19,
+      cacheWrite: undefined,
+      total: 39,
+    });
+  });
+
+  it("handles Kimi K2 prompt_tokens_details.cached_tokens field", () => {
+    // Kimi K2 uses automatic prefix caching and returns cached_tokens in prompt_tokens_details
+    const usage = normalizeUsage({
+      prompt_tokens: 1113,
+      completion_tokens: 5,
+      total_tokens: 1118,
+      prompt_tokens_details: { cached_tokens: 1024 },
+    });
+    expect(usage).toEqual({
+      input: 1113,
+      output: 5,
+      cacheRead: 1024,
+      cacheWrite: undefined,
+      total: 1118,
+    });
+  });
+
   it("returns undefined when no valid fields are provided", () => {
     const usage = normalizeUsage(null);
     expect(usage).toBeUndefined();