Fix mutable default in ListMemoryConfig (#6729)

### Fix mutable default in
[ListMemoryConfig](cci:2://file:///c:/Users/T2430514/Downloads/autogen/python/packages/autogen-core/src/autogen_core/memory/_list_memory.py:12:0-18:65)


[ListMemoryConfig](cci:2://file:///c:/Users/T2430514/Downloads/autogen/python/packages/autogen-core/src/autogen_core/memory/_list_memory.py:12:0-18:65)
used a shared empty list (`memory_contents: List[MemoryContent] = []`)
as its default, causing every
[ListMemory](cci:2://file:///c:/Users/T2430514/Downloads/autogen/python/packages/autogen-core/src/autogen_core/memory/_list_memory.py:21:0-171:79)
instance to share the same underlying list. This unexpected state
leakage let memories written in one instance silently surface in others,
breaking isolation and leading to hard-to-reproduce bugs.
Replaced the mutable default with a safe Pydantic
`Field(default_factory=list)`, ensuring each configuration—and thus each
[ListMemory](cci:2://file:///c:/Users/T2430514/Downloads/autogen/python/packages/autogen-core/src/autogen_core/memory/_list_memory.py:21:0-171:79)—gets
its own independent list.

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Co-authored-by: T2430514 <t2430514@gmail.com>
Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>