feat(spec): add message role

[matoushavlena]

Refs #141

This PR adds a role field to the Message schema in the OpenAPI specification.

Motivation

It's important to clearly track message originators within conversation history. Introducing the role field explicitly captures whether a message originated from a user or a specific agent, supporting the following scenarios:

• User ↔ single agent conversations
• User ↔ multiple agents interactions
• Agent ↔ agent communication

This addition maintains compatibility with standard conversational roles (user, assistant) widely adopted by chat-based LLM templates.

Next actions

• Update SDKs
• Update existing examples
• Update docs