Add legacy frontend compatibility for V3 node schemas

[Extraltodeus]

This PR adds backward compatibility for older frontends that don't support the V3 COMBO schema format introduced in recent ComfyUI versions.

Fixes #11010

Changes

• Adds convert_v3_to_v1_schema() function in server.py to convert V3 COMBO objects to V1 format
• Adds --legacy-frontend-compat command-line argument to enable the conversion
• When enabled, converts node schemas from V3 format (('COMBO', {'options': [...]})) to V1 format (([...], {})) in the /object_info endpoint

Implementation details

The conversion function processes all input types and:

• Detects V3-style COMBO specifications with an options dict
• Converts them to the legacy tuple format (options_list, metadata_dict)
• Preserves additional metadata from the V3 format in the second element of the tuple
• Passes through V1 format and other input types unchanged

This allows users running older frontends (custom nodes, external interfaces) to continue working with the latest ComfyUI backend without modification, activated via python main.py --legacy-frontend-compat.

[Kosinkadink]

Hey, thanks for the issue report and PR. The version of the frontend in that issue's reproduction is over a year old. In the spring of this year, COMBOs were given the option to use the "COMBO" type with the options provided in the dict, before V3 became a thing. Because of that, I don't think we will be merging this in. In the long term, we will also be updating backend code to convert V1 to V3 instead of the other way around how it's currently done.

The issue you show is not reproducible on any frontend version in the last 9 months, they support both.

In an upcoming PR based on this branch: https://github.com/comfyanonymous/ComfyUI/tree/v3-improvements , the backend will be updated to use the "COMBO" with options in the dict for all nodes in the backend to make it consistent - both definitions will still be supported by frtonend, the linked PR will simply change how the backend handles it as currently COMBO-style definition is not validated.