refactor(tests): Replace 'as any' type assertions in edge case tests

[AngusHsu]

Description

ESLint is generating warnings for the use of `as any` type assertions in edge case tests.

Location

`src/tests/low-coverage-countries.test.ts` lines 367-368 (and potentially other similar cases)

Current Code

```typescript
expect(validateNationalId('HUN', null as any).isValid).toBe(false);
expect(validateNationalId('ISL', undefined as any).isValid).toBe(false);
```

Warning Message

Unexpected any. Specify a different type
@typescript-eslint/no-explicit-any

Proposed Solution

Replace `as any` with `as unknown as string` to maintain type safety while still testing edge cases:

```typescript
expect(validateNationalId('HUN', null as unknown as string).isValid).toBe(false);
expect(validateNationalId('ISL', undefined as unknown as string).isValid).toBe(false);
```

Tasks

• Audit all test files for `as any` usage
• Replace with `as unknown as string` or appropriate type
• Verify all tests still pass
• Ensure ESLint warnings are resolved

Impact

• Low priority - these are warnings, not errors
• All checks currently pass
• Improves code quality and type safety

Labels

refactor, tests, code-quality, tech-debt