[Bug]: MemoryContent.mime_type enum degenerates to str after dump/load roundtrip (ListMemory)

What happened?

MemoryContent.mime_type accepts MemoryMimeType | str, and serializes any enum value to its string form via a field_serializer. There is no inverse validator on the way back in, so after a dump_component() / load_component() roundtrip (e.g. saving and restoring a ListMemory), the enum degenerates into a plain str. Any code that dispatches on content.mime_type == MemoryMimeType.TEXT (or is MemoryMimeType.X) silently changes behavior after a save/load.

This is a follow-on from #5688: the serializer added there fixed the "not JSON serializable" error but did not restore the enum on load.

Autogen-core code itself currently doesn't compare mime_type against the enum, but user code and autogen_ext memory implementations (e.g. semantic/chroma-based memories) that filter or dispatch by MIME type do — and they work before the roundtrip and break after it.

Repro

import asyncio
from autogen_core.memory import ListMemory, MemoryContent, MemoryMimeType

async def main():
    mem = ListMemory(name="t")
    await mem.add(MemoryContent(content="hello", mime_type=MemoryMimeType.MARKDOWN))

    cfg = mem.dump_component()
    print(repr(cfg.config["memory_contents"][0]["mime_type"]))   # 'text/markdown'

    loaded = ListMemory.load_component(cfg)
    print(type(loaded._contents[0].mime_type).__name__)          # str
    print(loaded._contents[0].mime_type == MemoryMimeType.MARKDOWN)  # False

asyncio.run(main())

Observed on autogen-core 0.7.5 and confirmed unfixed on main (python/packages/autogen-core/src/autogen_core/memory/_base_memory.py, the MemoryContent model has serialize_mime_type but no matching validator).

Expected behavior

mime_type should round-trip: if the serialized string matches a MemoryMimeType value, loading should restore the enum member, so loaded._contents[0].mime_type == MemoryMimeType.MARKDOWN holds after save/load.

Suggested fix

A small field_validator(mode="before") on mime_type that coerces a string matching an enum value back to MemoryMimeType (leaving arbitrary custom MIME-type strings as str):

@field_validator("mime_type", mode="before")
@classmethod
def _coerce_mime_type(cls, v: object) -> object:
    if isinstance(v, str):
        try:
            return MemoryMimeType(v)
        except ValueError:
            return v
    return v

Version

• autogen-core 0.7.5 (also reproduced against main source as of 2026-09-29)
• Python 3.13.9, macOS arm64