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

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`.

```ts
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"`);
});
});

```