fix(apigateway): duplicate methodResponses if the same array is reuse…

…d between addMethod calls (#26636)

Adding a new method to an API with `addMethod` and passing `methodOptions.methodResponses` was generating duplicate entries using `StepFunctionsIntegration.startExecution` as integration:

For example:
```
const integ = apigw.StepFunctionsIntegration.startExecution(stateMachine, integrationOptions);
api.root.addMethod('GET', integ, methodOptions);
api.root.addMethod('POST', integ, methodOptions);
```

Would generate (fails on deployment):
```
 "MethodResponses": [
     {
      "ResponseParameters": {
       "method.response.header.Access-Control-Allow-Origin": true
      },
      "StatusCode": "200"
     },
     {
      "ResponseModels": {
       "application/json": "Empty"
      },
      "StatusCode": "200"
     },
     {
      "ResponseModels": {
       "application/json": "Error"
      },
      "StatusCode": "400"
     },
     {
      "ResponseModels": {
       "application/json": "Error"
      },
      "StatusCode": "500"
     },
     {
      "ResponseModels": {
       "application/json": "Empty"
      },
      "StatusCode": "200"
     },
     {
      "ResponseModels": {
       "application/json": "Error"
      },
      "StatusCode": "400"
     },
     {
      "ResponseModels": {
       "application/json": "Error"
      },
      "StatusCode": "500"
     }
    ],
```

With this fix, it will keep only the specified `methodResponses`.

Also, the `integrationResponses` option in `StepFunctionsIntegration.startExecution` was not used by the corresponding `Integration`.
This fix will allow to use the specified option value.


Closes #26586.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk-testing/framework-integ/test/aws-apigateway/test/integ.stepfunctions-startexecution.js.snapshot/aws-cdk-aws-apigateway-stepfunctions-startexecution.assets.json

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