Use Ai::Conversation::Message directly in GraphQL read

What does this MR do and why?

• Adds duo_chat_read_directly_from_db feature flag to turn on/off the following changes:

  • Update ChatStorage#messages to return Ai::Conversation::Message.
  • For ChatMessagesResolver, instead of to_h every items, pass Ai::Conversation::Message directly to GraphQL for serialization
  • For Types::Ai::MessageType, make it able to handle both ChatMessage and Ai::Conversation::Message.
    • id has to be sourced from message_xid in Ai::Conversation::Message
    • thread has to be routed to object.thread instead of object['thread'] (this is a quirk of ActiveRecord).
    • errors has to be routed to error_details column (since errors is already reserved internally in ActiveRecord)
  • For Duo feedback, update message record directly

• Other behavior changes not affected by the feature flag

  • Polyfill Ai::Conversation::Message so it behaves a much like ChatMessage as possible

References

How to set up and validate locally

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Related to #501150 (closed)