Refactor Test Suite: Modularize and Expand Coverage

Improve the maintainability and reliability of the `@ramifyjs/core` test suite by splitting tests into module-specific files and closing gaps in coverage.

Key Objectives:

Module-wise Split: Decouple the monolithic test file into dedicated files for core modules
* collection.test.ts: CRUD, bulk operations, and index management.
* query.test.ts: Fluent API filtering, sorting, and results processing.
* ramify.test.ts: Store initialization and collection management.
* observer.test.ts: Subscription and notification logic.

Comprehensive Coverage: Ensure all edge cases are covered, specifically:
* Validation errors (e.g., non-primitive primary keys).
* Newer features like .modify() and the $allOf operator.
* Bulk operation side effects and observer notifications.