refactor: add stricter TypeScript types to mock-db helpers

- Add typed query builder interfaces (SelectQueryBuilder, InsertQueryBuilder,
  UpdateQueryBuilder) with proper method chaining types
- Add WhereResult, FromResult, OrderByResult, GroupByResult interfaces
- Add TableQuery interface for findFirst operations with typed params
- Update BillingDbConnection to use generics instead of `any`
- Add JSDoc comments with usage examples throughout
- Add OnInsertCallback, OnUpdateCallback types for tracking operations
- Add TrackedMockDbResult interface for better autocomplete
- Convert MockDbConfig from type to interface with documentation

All 104 billing tests pass with improved type safety.

Author: brandonkachen