Python: Add core utilities unit tests

[giles17]

Motivation and Context

This PR adds unit tests to improve coverage for the core utility modules (_memory.py, _serialization.py, _threads.py) as part of issue #3356 (achieving 85-90% unit test coverage for agent-framework-core).

Description

Added comprehensive unit tests for:

_memory.py (81% → 96%)

_serialization.py (81% → 94%)

_threads.py (85% → 98%)

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• [] Is this a breaking change? No

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
TOTAL	16249	2312	85%

report-only-changed-files is enabled. No files were changed during this commit :)

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3512	221 💤	0 ❌	0 🔥	1m 6s ⏱️

[Copilot]

Pull request overview

This PR adds comprehensive unit tests to improve coverage for three core utility modules (_memory.py, _serialization.py, _threads.py) in the Python agent-framework-core package, contributing to the goal of achieving 85-90% unit test coverage for the core package (issue #3356).

Changes:

• Added 4 new tests for ContextProvider base class default implementations in test_memory.py (coverage: 81% → 96%)
• Added 15 new tests for SerializationMixin edge cases including nested serialization, datetime handling, and dependency injection in test_serializable_mixin.py (coverage: 81% → 94%)
• Added 16 new tests for ChatMessageStore, AgentThread, and state classes edge cases in test_threads.py (coverage: 85% → 98%)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
python/packages/core/tests/core/test_memory.py	Added tests for ContextProvider base class default implementations (thread_created, invoked, async context manager behavior)
python/packages/core/tests/core/test_serializable_mixin.py	Added tests for advanced serialization scenarios including nested objects, datetime conversion, non-serializable handling, JSON deserialization, type identifiers, and dependency injection patterns
python/packages/core/tests/core/test_threads.py	Added tests for AgentThread initialization states, ChatMessageStore deserialization, state management edge cases, and exception handling