test: add comprehensive coverage for Termii SDK

[samdoghor]

This pull request introduces several improvements to the codebase, focusing on enhanced error handling, improved parameter validation, and the addition of comprehensive unit tests. It also includes minor dependency and version updates. The changes improve the reliability, maintainability, and test coverage of the project.

Error Handling Improvements:

• Replaced generic ValueError exceptions with more descriptive ClientConfigError exceptions in TermiiClient, providing clearer guidance and actionable error messages when required configuration values are missing. [1] [2]

Parameter Validation and Logic Enhancements:

• Improved channel and type parameter normalization and validation in send_message and send_bulk_message methods to ensure correct usage and clearer error messages, preventing misuse of WhatsApp and voice channels. [1] [2]
• Updated the phone number validation logic to immediately reject non-string inputs, making PhoneNumber.is_valid_phone_number more robust.

Testing and Test Infrastructure:

• Added extensive unit tests for TermiiClient configuration, HTTP request handling, and phone number value object, significantly increasing test coverage and reliability. [1] [2] [3]
• Introduced an optional test dependency group in pyproject.toml for easier test setup with pytest.

Other Updates:

• Updated the package version to 0.1.2 in termii_py/__init__.py.
• Improved file upload handling in RequestHandler.post_file by using a context manager to ensure files are properly closed after upload.