refactor: make mm_data_mooncake msgpack-native (drop PickleWrapper)

[mvanhorn]

Motivation

Follow-up to #28688, which moved IPC dataclasses to msgspec.Struct and added the opt-in msgpack transport (SGLANG_USE_PICKLE_IPC=0). Several fields still fall back to PickleWrapper. This handles Task 6 of #29465: mm_data_mooncake on TokenizedGenerateReqInput was typed Optional[PickleWrapper] and carried a pickled List[{"url": ..., "modality": Modality}] produced by MMReceiverBase._extract_url_data. Giving it a concrete msgspec type removes the whole-field pickle round-trip on the mooncake encode path. It is the most self-contained of the open tasks: one field, a known shape, and a single producer.

Refs #29465

Modifications

• Added a MooncakeMMUrlItem msgspec.Struct (url, modality) in io_struct.py and retyped mm_data_mooncake from Optional[PickleWrapper] to Optional[List[MooncakeMMUrlItem]]. url is typed Any because msgspec cannot express a union mixing str and bytes; every payload _extract_url_data produces (URL string, raw dict, or bytes) is a native msgpack type, so the field round-trips without a wrapper.
• Dropped mm_data_mooncake from wrap_pickle_fields() / unwrap_pickle_fields() on TokenizedGenerateReqInput. mm_inputs and time_stats are untouched.
• Updated the producer MMReceiverBase._extract_url_data to build MooncakeMMUrlItem instead of a dict, and switched every consumer in encode_receiver.py (mooncake RDMA dispatch, HTTP encode dispatch, gRPC encode, and _assign_items_by_modality) from ["url"]/["modality"] dict access to attribute access.
• Added test/registered/unit/managers/test_io_struct_msgpack.py covering msgpack round-trip of mm_data_mooncake: populated string URLs, a raw dict payload, None, empty list, and each Modality variant.

tokenizer_manager.py (passes the field through) and request_logger.py (lists it as a log-skip name) need no change.

Accuracy Tests

Not applicable: this is a transport type change for an internal IPC field and does not affect model outputs.

Speed Tests and Profiling

Not applicable to accuracy; the change removes a pickle encode/decode for this field on the mooncake path under msgpack IPC.

Checklist

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• Add unit tests according to the Run and add unit tests.

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CI States

Latest PR Test (Base): ❌ Run #28315034919
Latest PR Test (Extra): ❌ Run #28315034857

[gemini-code-assist]

Code Review

This pull request replaces the pickled dictionary representation of Mooncake multimodal URL data with a typed msgspec.Struct named MooncakeMMUrlItem. This allows native msgpack serialization and deserialization without pickle wrapping, improving performance and type safety. Corresponding updates were made in encode_receiver.py to use attribute access instead of dictionary access, and unit tests were added to verify the round-trip serialization of the new struct. There are no review comments, and I have no additional feedback to provide.

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[ShangmingCai]

Logic looks good to me, but why only refactor mm_data_mooncake?

CC: @liusy58 @ZhengWG for double-check.

[ShangmingCai]

@rainj-me, could you also help check this?

[ronhuafeng]

I validated the current PR head 95b01f7 for the Task 6 slice from #29465.

Local checks:

git diff --check HEAD^..HEAD

python3 -m py_compile \
  python/sglang/srt/managers/io_struct.py \
  python/sglang/srt/disaggregation/encode_receiver.py \
  test/registered/unit/managers/test_io_struct_msgpack.py

PYTHONPATH=python python3 -m pytest \
  test/registered/unit/managers/test_io_struct_msgpack.py -q

Results:

• diff whitespace check: passed
• py_compile for the changed Python files/test: passed
• focused msgpack unit test in a disposable lmsysorg/sglang:latest container: 5 passed, 25 warnings, 3 subtests passed in 27.52s

I did not run GPU/server validation because this PR only changes the typed IPC
representation and unit coverage for mm_data_mooncake; it does not touch model
execution or GPU kernels.

[rainj-me]

Please treat Any extremely careful since the decode will never deserialize it.

[hnyls2002]

Thanks @mvanhorn! The encode_receiver.py module this PR refactors was removed in the New EPD refactor (#30398), so the change no longer applies. Closing as obsolete — please reopen against the new disaggregation/encoder/ layout if the msgpack-native cleanup is still relevant.