Add client-managed chat history to RAO interactions

[carlesonielfa]

Description

Allows clients to supply the full conversation history on each request via a new chat_history field, enabling ephemeral sessions where the client owns conversation state rather than relying on server-side session storage.

• InteractionRequest and StartInteraction gain a chat_history: Optional[List[HistoryQuestionAnswer]] field (default None, backward compatible). If it is supplied, even as an empty list, it overrides any server-side memory
• QuestionMemory stores the client history and short-circuits context_history() and get_chat_history() to return it when set, overriding any server-side session history
• Extracted _qa_list_to_context_string / _qa_list_to_chat_messages module-level helpers to eliminate duplication across session memory classes
• Wired the field end-to-end: API → broker message → SessionManager.activate() → start_question() → engine.main()

[sc-14897]

How was this PR tested?

• Unit test covering the override and fallback behaviour (test_memory_chat_history.py)
• VCR integration test running rephrase + static context + summarize with client-supplied history, verifying the rephrased question incorporates the history and the final answer reflects it (test_chat_history_workflow.py)

[shortcut-integration]

This pull request has been linked to Shortcut Story #14897: Add chat history parameter in interaction.