feat(rpc): add service errors and retryability classification

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/rpc/test/errors.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, test } from "bun:test";
+import {
+  RPC_ERROR_CODES,
+  ServiceError,
+  failure,
+  isRetryableStatus,
+  success,
+} from "../src/errors.ts";
+
+describe("rpc errors", () => {
+  test("success and failure build the result shape", () => {
+    expect(success(42)).toEqual({ ok: true, value: 42 });
+    const f = failure(RPC_ERROR_CODES.denied, "Forbidden");
+    expect(f.ok).toBe(false);
+    if (!f.ok) {
+      expect(f.code).toBe("RPC_DENIED");
+      expect(f.retryable).toBe(false);
+    }
+  });
+
+  test("only transport failures are retryable", () => {
+    // 5xx and 429 are the callee saying "try again"; everything else is final.
+    expect(isRetryableStatus(500)).toBe(true);
+    expect(isRetryableStatus(503)).toBe(true);
+    expect(isRetryableStatus(429)).toBe(true);
+    expect(isRetryableStatus(400)).toBe(false);
+    expect(isRetryableStatus(403)).toBe(false);
+    expect(isRetryableStatus(404)).toBe(false);
+    expect(isRetryableStatus(200)).toBe(false);
+  });
+
+  test("a denial is never retryable", () => {
+    const f = failure(RPC_ERROR_CODES.denied, "Forbidden");
+    if (!f.ok) expect(f.retryable).toBe(false);
+  });
+
+  test("ServiceError carries a code and does not leak a cause into its message", () => {
+    const error = new ServiceError(RPC_ERROR_CODES.handler, "Something failed");
+    expect(error.name).toBe("ServiceError");
+    expect(error.code).toBe("RPC_HANDLER");
+    expect(error.message).toBe("Something failed");
+    expect(error.retryable).toBe(false);
+  });
+
+  test("toResult produces an opaque failure", () => {
+    const error = new ServiceError(RPC_ERROR_CODES.handler, "db password is hunter2");
+    const result = error.toResult({ exposeMessage: false });
+    if (!result.ok) {
+      expect(result.message).toBe("Internal error");
+      expect(result.message).not.toContain("hunter2");
+    }
+  });
+});