aws-apprunner-alpha: redesign `Service` construct to unify `Source.fromEcr()` and `Source.fromEcrPublic()` methods

### Describe the bug

The current design of the `Service` construct in `aws-apprunner-alpha` has significant inconsistencies, particularly between `Source.fromEcr` and `Source.fromEcrPublic`. This issue proposes a redesign to unify these methods into a single interface, dynamically determining the repository type (public or private) using regex. For context, see the related issue: [#32974](https://github.com/aws/aws-cdk/issues/32974#issuecomment-2600733863)

### Regression Issue

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

### Last Known Working CDK Version

_No response_

### Expected Behavior

see above

### Current Behavior

see above

### Reproduction Steps

see above

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.176.0

### Framework Version

_No response_

### Node.js Version

22.12.0

### OS

Ubuntu 24.04.1

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start by reading the aws-apprunner-alpha Service construct and comparing the Source.fromEcr() and Source.fromEcrPublic() interfaces. Review the discussion in related issue #32974, then clarify the unified interface and repository-type detection requirements before changing anything. Done means the redesign is agreed and both source paths follow the same interface.

Written by the indexing model from the issue text.