Preserve chat history when using 'agent.run'

[rjambrecic]

Why are these changes needed?

We should be able to use agent.run(... multiple times without losing chat history
e.g.

assistant = AssistantAgent(
    name="Assistant",
    llm_config=llm_config,
)

chat_result = assistant.run(
    message="what is two plus two?",
    max_turns=1,
    user_input=False,
)

user (to Assistant):

what is two plus two?

--------------------------------------------------------------------------------
Assistant (to user):

Two plus two equals four.

--------------------------------------------------------------------------------

And then send another message (without losing chat history):

chat_result = assistant.run(
    message="What was my last question?",
    max_turns=1,
    user_input=False,
)

user (to Assistant):

What was my last question?

--------------------------------------------------------------------------------
Assistant (to user):

Your last question was, "what is two plus two?"
--------------------------------------------------------------------------------

Related issue number

Closes #1006

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/agentchat/conversable_agent.py	75.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (8541b30) and HEAD (5d23a4a). Click for more details.

HEAD has 1169 uploads less than BASE

Flag	BASE (8541b30)	HEAD (5d23a4a)
3.13	84	0
ubuntu-latest	120	1
commsagent-discord	9	0
optional-deps	132	0
3.10	94	0
3.9	73	0
browser-use	3	0
core-without-llm	14	1
macos-latest	105	0
windows-latest	116	0
3.11	51	1
commsagent-slack	9	0
3.12	39	0
commsagent-telegram	9	0
crawl4ai	9	0
docs	6	0
graph-rag-falkor-db	6	0
interop	9	0
interop-crewai	9	0
interop-langchain	9	0
interop-pydantic-ai	9	0
jupyter-executor	9	0
twilio	9	0
retrievechat-pgvector	10	0
retrievechat	15	0
retrievechat-mongodb	10	0
retrievechat-qdrant	14	0
agent-eval	1	0
gpt-assistant-agent	3	0
anthropic	15	0
lmm	3	0
teachable	3	0
gemini	14	0
cerebras	14	0
retrievechat-couchbase	3	0
groq	14	0
llama-index-agent	3	0
websurfer	15	0
mistral	14	0
bedrock	14	0
long-context	3	0
ollama	14	0
cohere	15	0
swarm	14	0
reasoning	14	0
together	14	0

Files with missing lines	Coverage Δ
autogen/agentchat/conversable_agent.py	66.69% <75.00%> (-2.60%)	⬇️

... and 43 files with indirect coverage changes