IAM Role / Trust-relationships -trustpolicy required for access-grants-s3 cannot be created with CDK

### Describe the bug

Trying to create a trust-policy for a given role, like this:


cannot be created with CDK.

### Regression Issue

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

### Last Known Working CDK Version

CDK Version: 2.1002.0 (build 09ef5a0)

### Expected Behavior

the trust-policy for a given role, will contain the given statements


### Current Behavior

the trust-policy will not be included in the role.

### Reproduction Steps

try this code:
```
let servicePrincipal = new iam.ServicePrincipal('access-grants.s3.amazonaws.com');
servicePrincipal.addToAssumeRolePolicy(trustPolicy);
servicePrincipal.addToPolicy(new iam.PolicyStatement({
sid: 'AccessGrantsTrustPolicy',
effect: iam.Effect.ALLOW,
principals: [new iam.ServicePrincipal('access-grants.s3.amazonaws.com')],
actions: ['sts:AssumeRole', 'sts:SetSourceIdentity'],
conditions: {
StringEquals: {
'aws:SourceAccount': '9xxx2',
'aws:SourceArn': 'arn:aws:s3:eu-central-1:9xxx2:access-grants/default',
},
},
}));

this.context.properties.accessGrantsRole = new iam.Role(this, this.id4res('AccessGrantsRole'), {
assumedBy: servicePrincipal,
inlinePolicies: {
'AccessGrantsPolicy': accessGrantsPolicy,
},
});
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

CDK Version: 2.1002.0 (build 09ef5a0)

### Framework Version

_No response_

### Node.js Version

Node.js v22.12.0

### OS

Win11

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start by synthesizing the reproduced TypeScript IAM Role using ServicePrincipal, addToAssumeRolePolicy, and addToPolicy, then inspect the generated trust policy. Compare it with the expected statements for access-grants.s3.amazonaws.com; done means the synthesized role contains the supplied trust-policy statements.

Written by the indexing model from the issue text.