Add ChatRole enum for message history schema

Summary

Introduce a ChatRole enum for message history while preserving backward compatibility with string role inputs.

Level: Beginner

Current State

• ChatMessage.role is currently a plain str in redisvl/extensions/message_history/schema.py.
• Role handling is split between schema and base history validation logic.

Proposed Change

• Add a ChatRole enum (user, assistant, system, tool).
• Update ChatMessage to accept enum values and coerce valid strings.
• Align checks in redisvl/extensions/message_history/base_history.py.
• Export the enum from the message history module.

Definition of Done

• Enum-based role validation is in place.
• Existing string-based behavior remains valid (non-breaking).
• Unit tests cover valid coercion and invalid role errors.
• make format, make check-types, and tests pass.

Out of Scope

• Redesigning message history schemas beyond the role type.

Start with redisvl/extensions/message_history/schema.py and then compare the role validation in redisvl/extensions/message_history/base_history.py. Check the existing message-history tests, add coverage for valid coercion and invalid roles, and confirm backward-compatible string inputs with make format, make check-types, and the test suite.