fix(usage): read Responses-shape cached_tokens in extractUsageFromResponse

Non-streaming codex traffic recorded cached_tokens: 0 even when upstream
prompt caching worked. The Claude-format branch (which OpenAI Responses
usage also matches) never read input_tokens_details, and the OpenAI
branch ignored a top-level flat cached_tokens. Read both in both
branches; Responses prompts are cache-inclusive so canonicalizeUsage
passes the value through without folding. 5 new regression tests.

tests/unit/extract-usage-cache-shapes.test.js

@@ -0,0 +1,74 @@
+import { describe, it, expect, vi } from "vitest";
+
+// sever the DB import chain (usageDb -> @/lib/db/*) — not under test
+vi.mock("@/lib/usageDb.js", () => ({
+  saveRequestUsage: vi.fn(),
+  appendRequestLog: vi.fn(),
+  saveRequestDetail: vi.fn(),
+}));
+// and the stream/console-coloring utils that drag in the translator graph
+vi.mock("../../open-sse/utils/stream.js", () => ({
+  COLORS: {},
+  formatSSE: vi.fn(),
+}));
+
+import { extractUsageFromResponse } from "../../open-sse/handlers/chatCore/requestDetail.js";
+import { canonicalizeUsage } from "../../open-sse/utils/usageTracking.js";
+
+// The three real-world usage shapes and how extractUsageFromResponse() must
+// surface their cache-read count so canonicalizeUsage() produces a correct
+// cached_tokens. Regression for non-streaming codex/Responses traffic, where
+// cache reads were silently dropped and usage recorded cached_tokens: 0.
+describe("extractUsageFromResponse cache surfaces", () => {
+  it("surfaces OpenAI Responses input_tokens_details.cached_tokens", () => {
+    // codex / /v1/responses shape: prompt is cache-INCLUSIVE
+    const out = extractUsageFromResponse({
+      usage: { input_tokens: 25421, output_tokens: 5, total_tokens: 25426,
+               input_tokens_details: { cached_tokens: 24320 } },
+    });
+    expect(out.cached_tokens).toBe(24320);
+    expect(out.prompt_tokens).toBe(25421);
+    expect(out.cache_read_input_tokens).toBeUndefined();
+  });
+
+  it("canonicalizes Responses usage without double-counting the prompt", () => {
+    const extracted = extractUsageFromResponse({
+      usage: { input_tokens: 25421, output_tokens: 5,
+               input_tokens_details: { cached_tokens: 24320 } },
+    });
+    const out = canonicalizeUsage(extracted);
+    // inclusive prompt passes through unchanged; cache reported as subset
+    expect(out.prompt_tokens).toBe(25421);
+    expect(out.cached_tokens).toBe(24320);
+    expect(out.total_tokens).toBe(25426);
+    expect(out.cache_creation_input_tokens).toBe(0);
+  });
+
+  it("still folds genuine Claude exclusive cache (regression)", () => {
+    const extracted = extractUsageFromResponse({
+      usage: { input_tokens: 100, output_tokens: 50,
+               cache_read_input_tokens: 200, cache_creation_input_tokens: 30 },
+    });
+    expect(extracted.cached_tokens).toBeUndefined();
+    const out = canonicalizeUsage(extracted);
+    expect(out.prompt_tokens).toBe(330); // 100 + 200 + 30
+    expect(out.cached_tokens).toBe(200);
+    expect(out.cache_creation_input_tokens).toBe(30);
+  });
+
+  it("surfaces flat cached_tokens on the OpenAI branch (SSE-to-JSON shape)", () => {
+    const out = extractUsageFromResponse({
+      usage: { prompt_tokens: 300, completion_tokens: 10, cached_tokens: 240 },
+    });
+    expect(out.cached_tokens).toBe(240);
+  });
+
+  it("keeps nested prompt_tokens_details.cached_tokens working (regression)", () => {
+    const out = extractUsageFromResponse({
+      usage: { prompt_tokens: 300, completion_tokens: 10,
+               prompt_tokens_details: { cached_tokens: 240 } },
+    });
+    expect(out.cached_tokens).toBe(240);
+    expect(canonicalizeUsage(out).cached_tokens).toBe(240);
+  });
+});