refactor: Migrate getCountryIdFormat() to registry

[AngusHsu]

Description

Migrate the getCountryIdFormat() function from switch statement to registry lookup.

Parent Epic

#13 - Convert main dispatcher to registry pattern

Depends On

• feat: Implement validator registry #50 - Implement validator registry

Current State

• Incomplete switch statement
• Many countries missing format info

Target State

```typescript
export function getCountryIdFormat(countryCode: string): IdFormat | null {
const validator = registry.get(countryCode);
if (!validator) {
return null;
}
return validator.getFormat();
}
```

Tasks

• Add getFormat() to all validators
• Register format functions
• Replace switch with registry lookup
• Complete missing format info
• Remove old switch code

Acceptance Criteria

• All existing tests pass
• All countries have format info
• No breaking API changes

Labels

refactor, migration, format