feat: add MessageStore base class for group chat message threads

[muhammadrashid4587]

Summary

• Introduces a MessageStore abstract base class for storing message threads in group chat teams
• Provides ListMessageStore — an in-memory implementation with optional TTL-based message expiration
• Updates BaseGroupChatManager to accept an optional message_store parameter
• Maintains full backward compatibility — the legacy _message_thread list is kept in sync

Design

class MessageStore(ABC):
    async def add(self, messages: Sequence[ChatMessage]) -> None: ...
    async def get(self) -> List[ChatMessage]: ...
    async def clear(self) -> None: ...

class ListMessageStore(MessageStore):
    def __init__(self, *, ttl: timedelta | None = None): ...

The abstraction enables plugging in alternative backends (database, Redis, etc.) for long-running or persistent conversations.

Closes #6227

Test plan

• Existing tests pass — backward compatible (defaults to ListMessageStore)
• MessageStore and ListMessageStore exported from autogen_agentchat.teams
• TTL-based expiration in ListMessageStore.get() filters stale messages

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