misc: Update policy-guided safeguard to support initiate_group_chat API

[jiancui-research]

Why are these changes needed?

Update to support new API: initiate_group_chat. Previously, to use the policy-guided safeguard with initiate_chat it would be:

apply_safeguard_policy(groupchat_manager=manager, policy=policy, safeguard_llm_config=llm_config)
user_proxy.initiate_chat(manager, message=user_query)

To use with initiate_group_chat:

result, context, last_agent = initiate_group_chat(
    pattern=pattern,
    messages=user_query,
    max_rounds=10,
    safeguard_policy=policy,
    safeguard_llm_config=llm_config,
)

This PR updates the code & documentation to reflect the new recommended approach

Related issue number

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/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.

[randombet]

Look good in general.

@jiancui-research I have a few questions.
Does this change support run_group_chat method? Could you verify?
https://github.com/ag2ai/ag2/blob/main/autogen/agentchat/group/multi_agent_chat.py#L155

If yes, could you add an example in the notebook and update the documentation?
If not, you may address this in a separate PR.

Could you fix the pre-commit error as well?

[jiancui-research]

@randombet, thanks for your comment.

I double checked. The current implementation also supports run_groupchat. I updated the demo notebook and document to include this API!

[randombet]

LGTM

[codecov]

Codecov Report

❌ Patch coverage is 23.37662% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/agentchat/group/safeguards/enforcer.py	18.51%	40 Missing and 4 partials ⚠️
autogen/agentchat/group/safeguards/api.py	0.00%	6 Missing and 1 partial ⚠️
autogen/agentchat/group/multi_agent_chat.py	14.28%	6 Missing ⚠️
autogen/agentchat/group/group_tool_executor.py	77.77%	2 Missing ⚠️

Files with missing lines	Coverage Δ
autogen/agentchat/group/group_tool_executor.py	87.61% <77.77%> (-0.93%)	⬇️
autogen/agentchat/group/multi_agent_chat.py	29.16% <14.28%> (-1.61%)	⬇️
autogen/agentchat/group/safeguards/api.py	55.17% <0.00%> (-3.37%)	⬇️
autogen/agentchat/group/safeguards/enforcer.py	23.37% <18.51%> (-1.07%)	⬇️

... and 41 files with indirect coverage changes

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