feat(coder): persist token breakdown in arena decisions and schema

apps/coder/src/services/__tests__/arena-decisions.test.ts

@@ -162,6 +162,24 @@ describe('computeBenchmark', () => {
     expect(bench.durationMs).toBe(0);
     expect(bench.tokensPerSec).toBeNull();
   });
+
+  it('includes token breakdown when provided', () => {
+    const breakdown = {
+      system: 10,
+      user: 20,
+      assistant: 30,
+      tools: 40,
+      reasoning: 5,
+      total: 105,
+    };
+    const bench = computeBenchmark(t0, t1, 500, 'local', breakdown);
+    expect(bench.tokenBreakdown).toEqual(breakdown);
+  });
+
+  it('defaults token breakdown to null when omitted', () => {
+    const bench = computeBenchmark(t0, t1, 500, 'local');
+    expect(bench.tokenBreakdown).toBeNull();
+  });
 });
 
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