[Task] Identify and add missing test cases

[Apricot-S]

Summary

The current test suite does not fully cover all modules, code paths, and error conditions in the library.
This issue aims to identify missing test cases and add them to improve overall reliability and confidence during refactoring.

Scope

This task includes:

• Reviewing the existing test suite to determine what is missing
• Adding test cases for uncovered normal‑flow behavior across the library
• Adding test cases for edge cases and unusual inputs

Motivation

• Improve confidence in ongoing refactoring for v2.0.0
• Ensure that both normal and abnormal inputs are handled correctly
• Detect regressions earlier

Discussion points

• Which modules or functions currently lack coverage
• Which error conditions should be explicitly tested

Notes on abnormal‑case testing

This issue explicitly includes adding tests for:

• Invalid or malformed inputs
• Boundary conditions
• Cases where exceptions should be raised
• Unexpected or inconsistent internal states
• Inputs that violate assumptions but should still fail gracefully