Python: prepend_instructions_to_messages inverts instruction order on partial dedup

[ManoharPaturi]

Description

prepend_instructions_to_messages() in agent_framework/_types.py inverts instruction order when only part of the instruction list is already present as leading messages. The dedup added in #5051 prepends the non-matching remainder in front of the matched instructions, reordering the system prompt silently.

Reproduction

from agent_framework._types import Message, prepend_instructions_to_messages

msgs = [Message(role="system", contents=["First"])]
out = prepend_instructions_to_messages(msgs, ["First", "Second"])
# instruction order in output: ['Second', 'First']  <-- inverted

Expected ['First', 'Second'] — instruction sequence order must be preserved regardless of how much is already present.

Environment

agent-framework python main, Python 3.12

[github-actions]

🤖 Automated triage reproduction notes (agent-authored — trust but verify)

Agent analysis

Repro: python/packages/core/agent_framework/_types.py::prepend_instructions_to_messages at lines 2015-2025 reverses relative instruction order when only a leading prefix is already present as same-role messages. Minimal repro: pass [Message("system", ["First instruction"])] with instructions ["First instruction", "Second instruction"] and assert the result texts preserve that order; current output places the second instruction first.

• Failing test: python/packages/core/tests/core/test_types.py::test_prepend_instructions_preserves_order_with_partial_duplicate
• Files examined: python/packages/core/agent_framework/_types.py, python/packages/core/tests/core/test_types.py, python/packages/core/pyproject.toml, python/packages/openai/agent_framework_openai/_chat_completion_client.py, python/packages/ollama/agent_framework_ollama/_chat_client.py, python/packages/mistral/agent_framework_mistral/_chat_client.py
• Tests run: python/packages/core/tests/core/test_types.py::test_prepend_instructions_preserves_order_with_partial_duplicate, python/packages/core/tests/core/test_types.py -k prepend_instructions
• Current version: 1.18.0