IAM.Policy: policy.attachToRole is not respecting role mutability

### Describe the bug

policy.attachToRole() is overly granting policies to imported roles with mutable set to false.

### Regression Issue

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

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

policy.attachToRole() should respect the mutabliity of the imported role.

### Current Behavior

policy.attachToRole() does not respect the mutabliity of the imported role.

### Reproduction Steps

```
const importedRole = Role.fromRoleName(
this,
'LookUpRole',
'LookUpRole',
{ mutable: false },
);

const policyStatement = new PolicyStatement({
effect: Effect.ALLOW,
    actions: ['execute-api:Invoke'],
  resources: ["*"],
});

const policy = new ManagedPolicy(this, "policy", {
managedPolicyName: `testPolicy`,
description: 'an example policy',
statements: [
policyStatement
],
});

policy.attachToRole(importedRole);
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

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

2.257.0

### AWS CDK CLI version

2.1125.0

### Node.js Version

24

### OS

AL2

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start by tracing ManagedPolicy.attachToRole() with Role.fromRoleName() and the imported role's mutable: false option. Confirm the current behavior and identify the existing IAM policy or role tests covering attachment; done means an immutable imported role is not granted or modified by policy.attachToRole().

Written by the indexing model from the issue text.