What would you like to be added?

This feature request proposes to refactor existing helper functions related to content Part processing into a dedicated utility module and to add comprehensive unit tests for them.

Specifically, the following changes will be implemented:

1. Extraction of Utility Functions: The partToString and getResponseText functions, currently located in packages/core/src/utils/summarizer.ts, will be extracted into a new, dedicated module: packages/core/src/utils/partUtils.ts.
2. Comprehensive Unit Tests: A new test file, packages/core/src/utils/partUtils.test.ts, will be created to provide thorough unit test coverage for both partToString and getResponseText. These tests will cover various scenarios, including valid inputs,
   edge cases, and invalid inputs (using @ts-expect-error for expected type errors).
3. Module Renaming: The new utility module will be named partUtils.ts to align with existing project naming conventions for utility files (e.g., utils suffix instead of helpers).

Why is this needed?

This refactoring and addition of tests are needed for several key reasons:

1. Improved Code Organization and Separation of Concerns: By extracting partToString and getResponseText into their own module, we achieve a clearer separation of concerns. These functions are general-purpose utilities for handling Part objects, and
   their current co-location with summarization logic in summarizer.ts is not ideal. A dedicated partUtils.ts module makes the codebase more modular and easier to navigate.
2. Enhanced Reusability: Placing these utility functions in a standalone module makes them easily discoverable and reusable across different parts of the codebase that might need to process Part objects, without introducing unnecessary dependencies on
   the summarizer module.
3. Increased Test Coverage and Robustness: Adding dedicated unit tests for these functions ensures their correctness and robustness. This provides a safety net for future modifications and helps prevent regressions, especially for functions that handle
   various input types and edge cases.
4. Adherence to Project Conventions: Renaming the module to partUtils.ts ensures consistency with the project's established naming conventions for utility files, contributing to a more uniform and maintainable codebase.

Additional context

No response