chore: import upstream snapshot with attribution

tests/unit/chatcore-client-usage-buffer.test.ts

@@ -0,0 +1,78 @@
+// Characterization of applyClientUsageBuffer — the non-streaming usage buffer/estimate block
+// extracted from handleChatCore (chatCore god-file decomposition, #3501). Deps are injected so the
+// buffer-vs-estimate branch and the in-place mutation of translatedResponse.usage are observable.
+// Locks: usage present → buffer+filter; usage absent → estimate from content length; empty content
+// (length 2 from JSON.stringify("")) still estimates; the mutation target is translatedResponse.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+const { applyClientUsageBuffer } = await import(
+  "../../open-sse/handlers/chatCore/clientUsageBuffer.ts"
+);
+
+function makeDeps(overrides: Record<string, unknown> = {}) {
+  const calls = { buffer: [] as unknown[], estimate: [] as unknown[], filter: [] as unknown[] };
+  const deps = {
+    addBufferToUsage: (u: unknown) => {
+      calls.buffer.push(u);
+      return { ...(u as object), _buffered: true };
+    },
+    estimateUsage: (...a: unknown[]) => {
+      calls.estimate.push(a);
+      return { _estimated: true };
+    },
+    filterUsageForFormat: (u: unknown, _fmt: unknown) => {
+      calls.filter.push(u);
+      return { ...(u as object), _filtered: true };
+    },
+    ...overrides,
+  } as Parameters<typeof applyClientUsageBuffer>[3];
+  return { deps, calls };
+}
+
+test("usage present → buffer then filter, mutates in place", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = { usage: { prompt_tokens: 5 } };
+  applyClientUsageBuffer(resp, { messages: [] }, "openai", deps);
+  assert.equal(calls.buffer.length, 1);
+  assert.equal(calls.estimate.length, 0);
+  assert.equal((resp.usage as Record<string, unknown>)._buffered, true);
+  assert.equal((resp.usage as Record<string, unknown>)._filtered, true);
+});
+
+test("no usage but content present → estimate then filter", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = {
+    choices: [{ message: { content: "hello world" } }],
+  };
+  applyClientUsageBuffer(resp, { messages: [] }, "openai", deps);
+  assert.equal(calls.buffer.length, 0);
+  assert.equal(calls.estimate.length, 1);
+  assert.equal((resp.usage as Record<string, unknown>)._estimated, true);
+  assert.equal((resp.usage as Record<string, unknown>)._filtered, true);
+  // estimateUsage receives (body, contentLength, format)
+  const args = calls.estimate[0] as unknown[];
+  assert.equal(args[2], "openai");
+  assert.equal(typeof args[1], "number");
+});
+
+test("empty content → JSON.stringify('') length 2 > 0 still estimates", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = {};
+  applyClientUsageBuffer(resp, {}, "claude", deps);
+  // content "" → JSON.stringify("") = '""' length 2 → contentLength 2 > 0
+  assert.equal(calls.estimate.length, 1);
+  const args = calls.estimate[0] as unknown[];
+  assert.equal(args[1], 2);
+});
+
+test("content length is computed from choices[0].message.content", () => {
+  const { deps, calls } = makeDeps();
+  const resp: Record<string, unknown> = {
+    choices: [{ message: { content: "abc" } }],
+  };
+  applyClientUsageBuffer(resp, {}, "openai", deps);
+  // JSON.stringify("abc") = '"abc"' → length 5
+  const args = calls.estimate[0] as unknown[];
+  assert.equal(args[1], 5);
+});