Python: Use message cache in agent orchestrations

[TaoChenOSU]

Motivation and Context

Some agent threads don't support adding messages. An example of this is the copilot studio agent.

In an agent orchestration, an agent may receive multiple messages before it is asked to generate a response. The currently approach in the orchestrations would add new messages to an agent thread whenever an agent receives a message, making the copilot studio agent not compatible with some orchestrations.

Description

Instead of adding messages to the agent thread, this PR introduces a message cache to store messages temporarily before the agent is invoked.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
agents/orchestration
agent_actor_base.py	67	6	91%	33, 79, 96, 132, 158, 175
group_chat.py	179	9	94%	111–113, 115, 286–288, 290, 355
handoffs.py	186	13	93%	254–256, 258, 270, 285, 296–297, 302, 314–317
magentic.py	287	97	66%	122–124, 256, 379, 447, 450, 457, 461, 463, 490–495, 497, 502, 504, 510, 512, 517–518, 520–521, 527, 529–530, 533–534, 540, 548–549, 558, 561–562, 564–567, 570–571, 575–578, 580–581, 583, 585–591, 594–595, 602, 611–613, 615, 617, 625–626, 628, 633, 637–638, 640, 642–643, 647–648, 650, 653, 658–660, 663, 665, 679–680, 684–685, 687, 689, 691, 693, 702–706, 773–774
TOTAL	26423	4509	82%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3639	22 💤	0 ❌	0 🔥	1m 49s ⏱️