Demote system messages in handoff context

[DDDonut (daleselaji-dev)]

Problem

A HandoffMessage can carry arbitrary LLMMessage context from one participant to another. The receive path inserted that context into the target agent's model context without changing message roles, so peer-authored SystemMessage content reached the target model with system authority.

Root Cause

The handoff receive logic in AssistantAgent, CodeExecutorAgent, and SelectorGroupChatManager directly called model_context.add_message() for every context entry. Normal peer messages are converted to UserMessage, but handoff context bypassed that boundary.

Solution

Normalize handoff context at the receive boundary. System messages from peer-provided handoff context are converted to UserMessage(source="handoff"); assistant and function-result messages retain their existing representation for the supported tool-call handoff flow.

Changes

• Add a shared demote_handoff_context_message() conversion helper.
• Apply it at all three handoff context splice sites.
• Add a regression test proving injected system text is stored as user content while the handoff message remains ordered after it.

Testing

• Baseline: python -m pytest packages/autogen-agentchat/tests/test_messages.py -q — 9 passed.
• Focused regression/context tests: python -m pytest packages/autogen-agentchat/tests/test_messages.py packages/autogen-agentchat/tests/test_assistant_agent.py::TestAssistantAgentMessageContext -q — 12 passed.
• ruff check on all changed files — passed.
• ruff format --check on all changed files — passed.
• Package Pyright via uv run poe pyright for autogen-agentchat and autogen-core — 0 errors.
• Repository-wide Pyright is not green because unrelated optional autogen-ext dependencies are not installed (cv2, ffmpeg, whisper, chromadb, graphrag, and related diagnostics).
• git diff --check — passed.

Compatibility/Risk

Only peer-provided SystemMessage entries inside HandoffMessage.context change role, which prevents privileged-role injection at the receive boundary. Existing assistant/tool-result handoff context remains unchanged. No public constructor or serialized schema changes are introduced.

Notes for Reviewer

The conversion is shared so the AssistantAgent, CodeExecutorAgent, and selector receive paths cannot drift. The source="handoff" marker makes the trust-boundary conversion explicit in downstream model context. This is intentionally limited to receive-side role demotion; tool capability policy is outside this PR.

Linked Issue

Fixes #8238

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