[Partner Nodes] fix(GPT Image): make custom_width/custom_height optional

OpenAIGPTImageNodeV2 declared the gpt-image-2 custom size widgets as
required nested inputs, so a prompt that picked a preset size and omitted
them failed validation with required_input_missing. execute() already
reads both with a 1024 default, and the deprecated OpenAIGPTImage1 node
in the same file already marks them optional.

comfy_api_nodes/nodes_openai.py

@@ -747,6 +747,7 @@ class OpenAIGPTImageNodeV2(IO.ComfyNode):
                                     max=3840,
                                     step=16,
                                     tooltip="Used only when `size` is 'Custom'. Must be a multiple of 16.",
+                                    optional=True,
                                 ),
                                 IO.Int.Input(
                                     "custom_height",
@@ -755,6 +756,7 @@ class OpenAIGPTImageNodeV2(IO.ComfyNode):
                                     max=3840,
                                     step=16,
                                     tooltip="Used only when `size` is 'Custom'. Must be a multiple of 16.",
+                                    optional=True,
                                 ),
                                 IO.Combo.Input(
                                     "background",