feat: add findRand() method to Eloquent Builder

Add findRand() method for convenient random record retrieval that follows
the existing find* method family pattern. Supports single and multiple
record selection with consistent return types.

• Add findRand($count = 1, $columns = ['*']) method
• Returns TModel|null for single records (count = 1)
• Returns Collection for multiple records (count > 1)
• Maintains consistency with existing find* methods
• Includes comprehensive test coverage
• Improves developer experience for common random selection use case

Examples:

User::findRand()           // Single random user
User::findRand(5)          // Collection of 5 random users
Post::published()->findRand(3, ['title', 'slug'])  // 3 random published posts

Fixes common anti-pattern of User::all()->random() and provides
intuitive API for database-level random record selection.