Python: Bugfix: issue-3364: detect and preserve custom store state

[hombin]

Motivation and Context

Fix issue-3364: #3364

Description

Intelligently detect and preserve custom store state while maintaining backward compatibility with standard ChatMessageStore.

The key insight is to distinguish between:

1. Standard stores: Only have type and messages [expected] fields → Convert to ChatMessageStoreState object (existing behavior)
2. Custom stores: Have additional fields (e.g., redis_url, thread_id) → Keep as dictionary to preserve all data

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? If yes, add "[BREAKING]" prefix to the title of the PR.

Test Update Required

One test needs updating: test_init_with_chat_message_store_state_no_messages (line 448 in tests/core/test_threads.py)

Why: This test uses Redis store data (with custom fields like thread_id, redis_url) but expects the state to be converted to a ChatMessageStoreState object with .messages attribute. With our fix, custom store states are correctly preserved as dictionaries to retain all configuration.

[hombin]

@microsoft-github-policy-service agree

[hombin]

Hi @markwallace-microsoft , when you have a moment, could you please help review and approve this PR?
Let me know if anything needs to be updated — happy to adjust. Thanks a lot!