Updates to make the library applicable for wider range of use cases, and more stable.

[polina-c]

1. Remove ChatMessageRole: this library should not enforce the set of roles, that can vary from case to case. The role should be part of envelope in client's code.
2. Rename ChatMessage to Message, because this library can be used in chats and in other systems.
3. Use constants for repeating literals
4. Remove postfix 'Part' from part type literals, to reduce size of messages.
5. Extend test coverage for primitives.
6. Add test for example.
7. fix case of null in nameFromMimeType
8. Followed Gemini's advises below

[polina-c]

/gemini review

[polina-c]

@csells , how does it look?

Options for name for the class ChatMessage:

1. Message (I used it for now, but I am open)
2. MessageBody
3. MessageContent
4. Content

[gemini-code-assist]

Code Review

This pull request introduces several significant and beneficial changes to make the library more flexible and robust. Key changes include removing the enforced ChatMessageRole, renaming ChatMessage to Message, and using constants for JSON keys to reduce message size. The test coverage has been significantly improved, including a new test for the example, which is excellent. My review focuses on some opportunities to improve code clarity and leverage more modern Dart features, such as using const constructors for immutable classes and refactoring switch statements/expressions for better readability. Overall, this is a solid pull request that improves the library's design and usability.