fix(sglang): support msgspec-based ServerArgs (sglang >= 0.5.20)

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sglang 0.5.20 moved its config tier from dataclasses to msgspec.Struct
(sgl-project/sglang#38753). _apply_engine_args validates engine_args keys
via dataclasses.fields(ServerArgs), which raises TypeError there. That
call runs on every LoadModel, so no model loads at all on the sglang
backend once sglang >= 0.5.20 is installed, and the error surfaces as a
generic "Unexpected <class 'TypeError'>" that does not name the cause.

Introspect both shapes: msgspec structs carry their field names in
__struct_fields__, so key validation and the close-match suggestion keep
working, and older dataclass-based sglang stays supported.

Adds a test that pins the msgspec path with a stand-in, so it is covered
regardless of which sglang version is installed.

Signed-off-by: pos-ei-don <1822533+pos-ei-don@users.noreply.github.com>

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