allOf Fails to Handle Array of if/then as Chained else Conditions

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

5.4.0

### Plugin version

_No response_

### Node.js version

22.14.0

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

15.5

### Description

It appears that `allOf` cannot handle a sequence of `if/then` conditions as a chain of else clauses. This pattern would be extremely useful for scaling conditional validation logic, especially since libraries like AJV support it correctly.

When using `allOf` with an if/then block that includes array-based conditions, fast-json-stringify fails to serialize or apply logic as expected. AJV validates the same schema correctly.

When multiple if/then pairs are used inside allOf, only the first condition seems to be evaluated, and subsequent ones are ignored or misinterpreted.

### Link to code that reproduces the bug

```javascript
import fastJson from "fast-json-stringify";

const schema = {
type: "object",
properties: {
kind: {
enum: ["obj_1", "obj_2"],
},
},
allOf: [
{
if: {
properties: {
kind: { type: "string", const: "obj_1" },
},
},
then: {
type: "object",
properties: {
kind: { type: "string" },
prop_1: { type: "integer" },
},
},
},
{
if: {
properties: {
kind: { type: "string", const: "obj_2" },
},
},
then: {
type: "object",
properties: {
kind: { type: "string" },
prop_2: { type: "string" },
},
},
},
],
required: ["kind"],
};

doTest("obj 1", { kind: "obj_1", prop_1: 5 }, schema); // {"kind":"obj_1","prop_1":5} { ajv_valid: true }
doTest("obj 2", { kind: "obj_2", prop_2: "5" }, schema); // {"kind":"obj_2"} { ajv_valid: true }
doTest("obj 3", { kind: "obj_2", prop_2: 5 }, schema); // {"kind":"obj_2"} { ajv_valid: false }

function doTest(name, obj, schema) {
const stringify = fastJson(schema);
const validate = fastJson(obj, { mode: "debug" }).ajv.compile(schema);
const ajv_valid = validate(obj);

console.log(name, stringify(obj), { ajv_valid });
}
```

### Expected Behavior

`allOf` should evaluate each if/then block in sequence, effectively allowing for a chain of mutually exclusive conditional branches (as a substitute for else if logic).