feat: AgentChat.NET Group Chat Patterns: Selector, Swarm

[lokitoth]

Closes #5803

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.22%. Comparing base (7d17b22) to head (3b07b5a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5838      +/-   ##
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- Coverage   75.65%   70.22%   -5.43%     
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  Files         189      262      +73     
  Lines       12784    14762    +1978     
  Branches        0      243     +243     
==========================================
+ Hits         9672    10367     +695     
- Misses       3112     4205    +1093     
- Partials        0      190     +190     

Flag	Coverage Δ
unittests	70.22% <ø> (-5.43%)	⬇️

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[Copilot]

PR Overview

This pull request introduces two new group chat patterns for AgentChat.NET: Swarm and Selector, enhancing the framework's capabilities for dynamic group conversations.

• Added a Swarm group chat manager that selects speakers based on recent handoff messages.
• Implemented a Selector group chat manager using a customizable prompt and selector function.
• Updated the PromptTemplate to support formatted message templates.

Reviewed Changes

File	Description
dotnet/src/Microsoft.AutoGen/AgentChat/GroupChat/SwarmGroupChat.cs	Implements Swarm group chat selection logic with minor error message text.
dotnet/src/Microsoft.AutoGen/AgentChat/Abstractions/PromptTemplate.cs	Provides templated string formatting for chat prompts.
dotnet/src/Microsoft.AutoGen/AgentChat/GroupChat/SelectorGroupChat.cs	Implements Selector group chat logic with customizable prompt and selector function.

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (2)

dotnet/src/Microsoft.AutoGen/AgentChat/GroupChat/SwarmGroupChat.cs:47

• [nitpick] Consider changing 'handoff messages' to 'handoff message' for grammatical correctness.

throw new InvalidOperationException("The first participant must be able to produce a handoff messages.");

dotnet/src/Microsoft.AutoGen/AgentChat/GroupChat/SelectorGroupChat.cs:140

• [nitpick] The word 'candidaate' appears to be misspelled; it should be 'candidate'. Additionally, confirm that the initializer syntax '[..' is intentional or replace it with valid C# syntax.

IEnumerable<string> participants = [.. from candidaate in this.groupOptions.Participants.Values

[rysweet]

so awesome! looks like we have tests for this in python - let's port them - will help as soon as I can