Allow STRING link to connect to converted COMBO socket

[huchenlei]

As COMBO widget is essentially selecting a specific string from a set of strings, we should allow connection of STRING link to converted COMBO socket for more dynamic workflow manipulation, i.e. dynamically generate a COMBO input option from other nodes.

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[huchenlei]

Ref: If invalid combo value is passed at runtime, the backend will report error:

[JorgeR81]

So, we could just connect a "String" node and it would get the COMBO list, automatically ?

And if we unplug the wire, would the node still "remember" the COMBO list ?

If the COMBO list it's not "forgotten", it may be possible to then connect cg-use-everywhere nodes, right ?

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Example:

It would work like this "Preprocessor Selector" node, in comfyui_controlnet_aux.
This node was added essentially to work with cg-use-everywhere.

It's meant to replace the Primitive COMBO, which cannot be connected to cg-use-everywhere nodes.
Fannovel16/comfyui_controlnet_aux#291

This means that ComfyUI would create these type of nodes automatically, as needed ?

[huchenlei]

There are some backend logic change required, as currently converted combo type socket is not being correctly validated in the backend.