fix(core): restore MemoryMimeType enum on mime_type after component roundtrip

[JingHao-Leon]

What happened?

MemoryContent.mime_type accepts MemoryMimeType | str and serializes enum values to their string form via field_serializer (added in #5688 to fix "Object of type MemoryMimeType is not JSON serializable"). Nothing restored the enum on the way back in, so after a dump_component() / load_component() roundtrip (e.g. saving and restoring a ListMemory), mime_type degenerated to a plain str:

mem = ListMemory(name="t")
await mem.add(MemoryContent(content="hello", mime_type=MemoryMimeType.MARKDOWN))
config = mem.dump_component()          # 'text/markdown'
loaded = ListMemory.load_component(config)
loaded._contents[0].mime_type == MemoryMimeType.MARKDOWN   # False (was str)

Any code that dispatches on content.mime_type == MemoryMimeType.TEXT (user code, or autogen_ext memory implementations) works before a save/load and silently changes behavior after it.

Fixes #8293 (which has the full repro and version details).

Are you a new contributor?

• Yes, this is my first PR (aside from fix(tools): persist strict flag in FunctionTool component config #8292, currently open)

How did you test it?

• New regression test test_list_memory_mime_type_roundtrip_restores_enum in python/packages/autogen-core/tests/test_memory.py: dumps a ListMemory holding a MemoryMimeType.MARKDOWN item, loads it back, asserts the enum member is restored and that a custom MIME string stays str.
• pytest tests/test_memory.py → 11 passed (10 existing + 1 new) with the fix; the new test fails on main without it.
• Repro from [Bug]: MemoryContent.mime_type enum degenerates to str after dump/load roundtrip (ListMemory) #8293 now prints MemoryMimeType with equality True.