fixing handoff history conflicts

[jhills20]

• Prevented handoff input filters from running when conversation_id/previous_response_id are used
  and automatically disable history nesting in that mode to keep the SDK in sync with server-managed
  conversations (with a clear error/warning instead of silent API failures).
• When nesting is enabled without server-managed conversations, the next agent now receives only the
  synthesized message (no duplicated raw items) while the full run history is
  still retained for results.
• Added coverage for the new server-conversation rules and documented the limitation in docs/
  handoffs.md.

[ihower]

Link to the issue for context: #2151

[github-actions]

This PR is stale because it has been open for 10 days with no activity.