aws-cdk-lib: CustomResource onEvent lambda function uses runtime nodejs22.x which is not latest

### Describe the bug

We use cdk-nag to check our code. With latest version of aws-cdk-lib, this is failing with error

```
AwsSolutions-L1: The non-container Lambda function is not configured to use the latest runtime version. Use the latest available runtime for the targeted language to avoid technical debt. Runtimes specific to a language or framework version are deprecated when the version reaches end of life. This rule only applies to non-container Lambda functions.
```

where we have a custom resource, as the lambda function created for onEvent uses runtime nodejs22.x, and not the latest node version.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

2.221.1

### Expected Behavior

runtime of onevent lambda should use latest node version

### Current Behavior

runtime of onevent lamba use nodejs22.x

### Reproduction Steps

This code can be used to reproduce it

```
import {App, Stack, StackProps} from "aws-cdk-lib"
import {Duration, CustomResource} from "aws-cdk-lib"
import {Function, Runtime, Code} from "aws-cdk-lib/aws-lambda"
import {Provider} from "aws-cdk-lib/custom-resources"

export class ErrorStack extends Stack {
public constructor(scope: App, id: string, props: StackProps) {
super(scope, id, props)
// create the delay Lambda function with inline Python code
const myFunction = new Function(this, "MyFunction", {
runtime: Runtime.PYTHON_3_12,
handler: "index.handler",
timeout: Duration.minutes(15),
description: "foo",
code: Code.fromInline(`
import cfnresponse

def handler(event, context):
cfnresponse.send(event, context, cfnresponse.SUCCESS, response, f"Waiter-{id}")
`)
})

// create the custom resource provider
const provider = new Provider(this, "myProvider", {
onEventHandler: myFunction
})

// create the custom resource that triggers the delay
new CustomResource(this, "MyCustomResource", {
serviceToken: provider.serviceToken
})

}
}
```

When you run cdk-synth and look at the cloudformation template, notice the runtime for the onEvent lambda is nodejs22.x

```
...
"myProviderframeworkonEventAB595BBA": {
"Type": "AWS::Lambda::Function",
"Properties": {
"Code": {
"S3Bucket": {
"Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-eu-west-2"
},
"S3Key": "bdc104ed9cab1b5b6421713c8155f0b753380595356f710400609664d3635eca.zip"
},
"Description": "AWS CDK resource provider framework - onEvent (ErrorStack/myProvider)",
"Environment": {
"Variables": {
"USER_ON_EVENT_FUNCTION_ARN": {
"Fn::GetAtt": [
"MyFunction3BAA72D1",
"Arn"
]
}
}
},
"Handler": "framework.onEvent",
"LoggingConfig": {
"ApplicationLogLevel": "FATAL",
"LogFormat": "JSON"
},
"Role": {
"Fn::GetAtt": [
"myProviderframeworkonEventServiceRole94BD41C2",
"Arn"
]
},
"Runtime": "nodejs22.x",
"Timeout": 900
},
...
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

2.230.0

### AWS CDK CLI version

2.1033.0

### Node.js Version

24.11.1

### OS

linux

### Language

TypeScript

### Language Version

Typescript 5.9.3

### Other information

_No response_

Start with the TypeScript reproduction's Provider and onEventHandler path, then run cdk-synth and inspect the generated CloudFormation template. The change is done when the framework-created onEvent Lambda uses the latest intended Node.js runtime instead of nodejs22.x, while the supplied user function remains unchanged.

Written by the indexing model from the issue text.