refactor(open-sse): usage.js dispatcher switch → USAGE_HANDLERS registry

Gộp switch 13 nhánh getUsageForProvider thành 1 registry object
(provider → handler), mỗi handler giữ nguyên signature/args qua ctx.
Behavior giữ nguyên (ollama vẫn chỉ nhận accessToken như cũ).
Thêm tests/unit/usage-dispatch.test.js guard dispatch.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/usage-dispatch.test.js

@@ -0,0 +1,39 @@
+// Guards the refactored USAGE_HANDLERS dispatch: unsupported → message, supported → routed.
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// Stub network so handlers don't hit real APIs; each call resolves an empty 200.
+vi.mock("../../open-sse/utils/proxyFetch.js", () => ({
+  proxyAwareFetch: vi.fn(async () => ({
+    ok: true,
+    status: 200,
+    json: async () => ({}),
+    text: async () => "{}",
+  })),
+}));
+
+const load = () => import("../../open-sse/services/usage.js");
+const SUPPORTED = [
+  "github", "gemini-cli", "antigravity", "claude", "codex", "kiro",
+  "qoder", "qwen", "iflow", "ollama", "glm", "glm-cn",
+  "minimax", "minimax-cn", "vercel-ai-gateway",
+];
+
+describe("usage dispatch", () => {
+  beforeEach(() => vi.clearAllMocks());
+
+  it("unsupported provider → not-implemented message", async () => {
+    const { getUsageForProvider } = await load();
+    const res = await getUsageForProvider({ provider: "totally-unknown" });
+    expect(res).toEqual({ message: "Usage API not implemented for totally-unknown" });
+  });
+
+  it("every supported provider routes to its handler (no fallback message)", async () => {
+    const { getUsageForProvider } = await load();
+    for (const provider of SUPPORTED) {
+      const res = await getUsageForProvider({ provider, accessToken: "t", apiKey: "k" });
+      // Routed handler must return an object and never the unsupported fallback
+      expect(res, `${provider} routed`).toBeTypeOf("object");
+      expect(res?.message).not.toBe(`Usage API not implemented for ${provider}`);
+    }
+  });
+});