Map "MODEL_3D" input type to MultiType socket (mirror upstream union)

Follow-up to PR #20: the `MODEL_3D` input wire type now resolves to
an `IO.MultiType.Input(types=[IO.File3DGLB, IO.File3DOBJ,
IO.File3DFBX, IO.File3DAny])` socket — symmetric counterpart of the
existing `"MODEL_3D"` entry in `_OUTPUT_CLASSES` that maps to
`IO.File3DGLB.Output`.

Previously the descriptor wire type `"MODEL_3D"` fell through to the
opaque `IO.Custom("MODEL_3D").Input(...)` socket, which is a
single-comfytype socket the frontend only allows connections from
identically-tagged outputs. Real upstream 3D sources expose
different comfytypes:

- `Load3D` outputs `IO.File3DAny` (comfytype `FILE_3D`).
- Meshy / Hunyuan3D / Tripo / Rodin partner nodes output
  `IO.File3DGLB` (comfytype `FILE_3D_GLB`) via the existing
  `_OUTPUT_CLASSES["MODEL_3D"]` mapping.
- Meshy's dual `FILE_3D_FBX` output socket emits `FILE_3D_FBX`.

None of these connect to `IO.Custom("MODEL_3D")`. The upstream
Tencent Hunyuan3D MODEL_3D-input nodes (`TencentModelTo3DUVNode` /
`Tencent3DTextureEditNode` / `Tencent3DPartNode` /
`TencentSmartTopologyNode`) all use `IO.MultiType.Input(types=[...])`
to accept the union; mirroring that union here keeps the proxy
socket compatible with every existing 3D-emitting node.

`get_io_type()` on `IO.MultiType.Input` returns the comma-joined
inner type IDs (e.g. `"FILE_3D_GLB,FILE_3D_OBJ,FILE_3D_FBX,FILE_3D"`)
— matches the convention SxS workflows already use for their
`SaveGLB` connector inputs.

Per-node format validation (e.g. 3DTextureEdit's FBX-only
enforcement, 3DPart's FBX-only enforcement) is done server-side in
each provider's `execute()` against the inline envelope's `format`
extra; this socket-level filter is purely about frontend connection
validity.

Unblocks the paired comfy-rnp-server PR (`TencentModelTo3DUVNode_RNP`
+ `Tencent3DPartNode_RNP`) which uses the `["MODEL_3D", {}]` wire
shape.

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019e362f-db40-7338-8ca4-99f617d89812

proxy_node.py

@@ -695,6 +695,30 @@ def _build_input(name: str, io_type: Any, options: dict[str, Any], optional: boo
         return IO.Audio.Input(name, **primitive_common)
     if io_type == "MASK":
         return IO.Mask.Input(name, **primitive_common)
+    if io_type == "MODEL_3D":
+        # 3D MODEL input: symmetric counterpart of the ``"MODEL_3D"``
+        # entry in ``_OUTPUT_CLASSES`` above. The Tencent Hunyuan3D
+        # MODEL_3D-input nodes (``TencentModelTo3DUVNode`` /
+        # ``Tencent3DTextureEditNode`` / ``Tencent3DPartNode`` /
+        # ``TencentSmartTopologyNode``) all declare their model_3d
+        # input as ``IO.MultiType.Input(types=[IO.File3DGLB,
+        # IO.File3DOBJ, IO.File3DFBX, IO.File3DAny])`` so the socket
+        # accepts any upstream 3D source: ``Load3D``'s ``File3DAny``
+        # output, Meshy / Hunyuan3D / Tripo / Rodin's per-format
+        # outputs (``FILE_3D_GLB`` / ``FILE_3D_OBJ`` / ``FILE_3D_FBX``),
+        # etc. Mirror that union here so the proxy socket accepts the
+        # same set of comfytypes upstream does. Per-node format
+        # validation (e.g. 3DTextureEdit's FBX-only enforcement) is
+        # done server-side in the provider's ``execute()`` against
+        # the inline envelope's ``format`` extra; this socket-level
+        # filter is purely about frontend connection validity.
+        return IO.MultiType.Input(
+            name,
+            types=[
+                IO.File3DGLB, IO.File3DOBJ, IO.File3DFBX, IO.File3DAny,
+            ],
+            **primitive_common,
+        )
     if io_type == "DYNAMIC_COMBO":
         # Wire shape: ``["DYNAMIC_COMBO", {"options": [{"key": str,
         # "inputs": [[name, io_type, opts], ...]}, ...], "tooltip": ...}]``.