Cabidela mutates schema and remove $defs when using `applyDefaults: true`

[mattallty]

Hello,

And thanks for this library!

I noticed a serious bug where Cabidela mutates the original schema when validating data.

Here is the bug demo where $defs are completely removed from the original schema when using applyDefaults: true.

import { describe, expect, test } from "vitest";
import { Cabidela } from "@cloudflare/cabidela";

describe("Cabidela schema validation bug with applyDefaults and $defs", () => {
  const schema = {
    $id: "http://example.com/schemas/main",
    type: "object",
    properties: {
      name: { type: "string" },
      balance: { $ref: "$defs#/balance" },
    },
    required: ["name", "balance"],
    $defs: {
      balance: {
        type: "object",
        properties: {
          currency: { type: "string" },
          amount: { type: "number" },
        },
      },
    },
  };

  const schemaBeforeValidationSerialized = JSON.stringify(schema);

  // Using applyDefaults: true should not mutate the original schema, but it does
  const validator = new Cabidela(schema, {
    applyDefaults: true,
  });

  // PASS
  test("validation passes for valid data", () => {
    const validData = { name: "Alice", balance: { currency: "USD", amount: 100 } };
    expect(validator.validate(validData)).toBe(true);
  });

  const schemeAfterValidationSerialized = JSON.stringify(schema);

  // PASS
  test("schemaBeforeValidationSerialized contains $defs", () => {
    expect(schemaBeforeValidationSerialized).toContain(`"$defs"`);
  });

  // FAILING TEST: This test fails because the schema is mutated by Cabidela and $defs disappears :(
  test("schemaAfterValidationSerialized still contains $defs", () => {
    expect(schemeAfterValidationSerialized).toContain(`"$defs"`);
  });
});