[FEATURE] Option to emit group chat manager messages in AgentChat

[SongChiYoung]

Why are these changes needed?

Add an option emit_team_events to BaseGroupChat to emit events from group chat manager through run_stream.
SpeakerSelectedEvent from group chat speaker selection.

Related issue number

Closes #6161

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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.

[SongChiYoung]

@ekzhu
This PR adds support for emitting SpeakerSelectedEvent during group chat speaker selection.

I'm wondering if you'd like this to be extended to emit other internal team events as well.
Are there any specific types of events you'd want to be exposed at this level?

[ekzhu]

Looks good let's start with this.

[codecov]

Codecov Report

Attention: Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.

Project coverage is 77.49%. Comparing base (a4a16fd) to head (d86a01c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...p_chat/_magentic_one/_magentic_one_orchestrator.py	25.00%	3 Missing ⚠️
...utogen-agentchat/src/autogen_agentchat/messages.py	85.71%	1 Missing ⚠️
...chat/teams/_group_chat/_base_group_chat_manager.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6303      +/-   ##
==========================================
+ Coverage   77.48%   77.49%   +0.01%     
==========================================
  Files         202      202              
  Lines       14717    14742      +25     
==========================================
+ Hits        11403    11424      +21     
- Misses       3314     3318       +4     

Flag	Coverage Δ
unittests	77.49% <90.19%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[SongChiYoung]

Hey @ekzhu,

I just saw the changes you pushed — the added unit tests, improved comments, and other refinements… I really appreciate the extra effort you put in. 🙏

Honestly, I feel a bit bad for not catching those things myself — I’m still learning and trying to get better at writing more polished PRs from the start. Your improvements are super helpful, and I’m grateful for the care you put into them.

Thanks again — I’ll keep iterating and do my best to make your review experience smoother next time.