Python: Introduce the agent chat history reducer

[moonbox3]

Motivation and Context

The SK Python framework has been missing the ability to configure a chat history reducer of type ChatHistoryTruncationReducer and ChatHistorySummarizationReducer which have existed in the .Net SK Agent framework for some time.

Description

This PR:

• Introduces the chat history reducer functionality in Python -- both the ChatHistoryTruncationReducer and ChatHistorySummarizationReducer.
• Add unit tests for code coverage.
• Adds a sample Chat Completion History Reducer to show how to configure both reducers and what each parameter does.
• Updates the Agent SelectionStrategy, KernelFunctionSelectionStrategy and KernelFunctionTermination strategy to use the reducer.
  • Additionally updates the classes above to use a new select_agent abstract method so that one can define an initial agent to run in a particular scenario.
• Closes Python Agents: ChatHistoryReducer #7969

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
semantic_kernel/agents
agent.py	44	10	77%	48–57, 70–71
semantic_kernel/agents/history
chat_history_reducer_extensions.py	30	4	87%	27, 74, 83–85
chat_history_summarization_reducer.py	79	6	92%	94, 127–130, 136–137, 148
chat_history_truncation_reducer.py	32	3	91%	39, 56–59
semantic_kernel/agents/strategies/selection
kernel_function_selection_strategy.py	54	1	98%	62
selection_strategy.py	18	1	94%	41
sequential_selection_strategy.py	27	1	96%	73
semantic_kernel/agents/strategies/termination
kernel_function_termination_strategy.py	44	1	98%	54
TOTAL	16892	1805	89%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3025	4 💤	0 ❌	0 🔥	1m 24s ⏱️