Update version to 0.8.15 and enhance error handling in agent execution and message management

[kryvokhyzha]

This pull request focuses on improving the reliability and robustness of the add_messages and clear_history endpoints, as well as fixing issues related to handling message lists (especially [-1] indexing). These changes ensure better error handling and data consistency across the agent toolkit service.

Endpoints and API robustness:

• Improved error handling in the add_messages and clear_history endpoints to gracefully handle missing or empty message histories, returning appropriate HTTP errors when necessary.
• Enhanced the add_messages endpoint to include all relevant message fields (type, content, tool_call_id, tool_calls), and to exclude fields with None values for cleaner data storage.
• Updated the history endpoint to safely retrieve messages using .get("messages", []), preventing errors when the key is missing.

Agent response handling:

• Added validation to ensure agent responses contain events and messages before accessing the last message, raising clear errors if not. This prevents IndexError when handling [-1] on empty lists.

Versioning and documentation:

• Bumped the project version to 0.8.15 and updated the CHANGELOG.md to reflect these fixes. [1] [2]