Complete context menu migration for Load3D, Preview3D, and SaveGLB (#6454)

This pull request refactors how export menu options are added to
3D-related nodes, updating the API to use a new `getNodeMenuItems` hook
and simplifying the menu item creation logic. The changes improve
consistency and maintainability across extensions handling 3D nodes, and
clarify the expected return types for menu item hooks.

**Refactoring and API updates for node menu items:**

* Replaced usage of the legacy `createExportMenuOptions` function with
the new `createExportMenuItems` function in `load3d.ts`, `saveMesh.ts`,
and related imports, aligning all 3D node extensions to the new API.
[[1]](diffhunk://#diff-a5c612d9322ca4cbbeda097d13e2fa1ef017d4c3076d23fc228afee5a79a56e3L6-R12)
[[2]](diffhunk://#diff-7dede72060130d77ce5191fc86d115bd9b93311cb0438400730d8e20b2aa8e43L4-R7)
* Introduced and implemented the `getNodeMenuItems` hook in the
extension registration for `Load3D`, `Preview3D`, and `SaveGLB` nodes,
ensuring export menu items are only shown for the appropriate node
types.
[[1]](diffhunk://#diff-a5c612d9322ca4cbbeda097d13e2fa1ef017d4c3076d23fc228afee5a79a56e3R293-R302)
[[2]](diffhunk://#diff-a5c612d9322ca4cbbeda097d13e2fa1ef017d4c3076d23fc228afee5a79a56e3R521-R530)
[[3]](diffhunk://#diff-7dede72060130d77ce5191fc86d115bd9b93311cb0438400730d8e20b2aa8e43R48-R57)
* Removed assignment of `getExtraMenuOptions` to nodes, fully migrating
to the new menu item hook approach for context menus.
[[1]](diffhunk://#diff-a5c612d9322ca4cbbeda097d13e2fa1ef017d4c3076d23fc228afee5a79a56e3L301-L302)
[[2]](diffhunk://#diff-a5c612d9322ca4cbbeda097d13e2fa1ef017d4c3076d23fc228afee5a79a56e3L548-L549)
[[3]](diffhunk://#diff-7dede72060130d77ce5191fc86d115bd9b93311cb0438400730d8e20b2aa8e43L64-L67)

**Menu item creation logic simplification:**

* Refactored `createExportMenuOptions` to `createExportMenuItems` in
`exportMenuHelper.ts`, changing it to directly return an array of menu
items (with separators) instead of a function, simplifying its usage and
reducing boilerplate.
[[1]](diffhunk://#diff-42404da1a87a52d304371a13d5f021bdad837765b94d86d186abb2d99a8cb707L14-R22)
[[2]](diffhunk://#diff-42404da1a87a52d304371a13d5f021bdad837765b94d86d186abb2d99a8cb707L59-R58)

**Type and documentation improvements:**

* Updated the `ComfyExtension` interface in `comfy.ts` to clarify that
menu item hooks (`getCanvasMenuItems`, `getNodeMenuItems`) now return
arrays that may include `null` values as separators, improving type
safety and documentation.

See before and after below
<img width="1459" height="897" alt="Screenshot 2025-10-30 at 07 08 04"
src="https://github.com/user-attachments/assets/ec4464c9-f733-4b4c-87c4-bb5060ccaa68"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6454-Complete-context-menu-migration-for-Load3D-Preview3D-and-SaveGLB-29c6d73d3650819995b4c4dc1582cd86)
by [Unito](https://www.unito.io)

src/types/comfy.ts

@@ -140,16 +140,16 @@ export interface ComfyExtension {
   /**
    * Allows the extension to add context menu items to canvas right-click menus
    * @param canvas The canvas instance
-   * @returns An array of context menu items to add
+   * @returns An array of context menu items to add (null values represent separators)
    */
-  getCanvasMenuItems?(canvas: LGraphCanvas): IContextMenuValue[]
+  getCanvasMenuItems?(canvas: LGraphCanvas): (IContextMenuValue | null)[]
 
   /**
    * Allows the extension to add context menu items to node right-click menus
    * @param node The node being right-clicked
-   * @returns An array of context menu items to add
+   * @returns An array of context menu items to add (null values represent separators)
    */
-  getNodeMenuItems?(node: LGraphNode): IContextMenuValue[]
+  getNodeMenuItems?(node: LGraphNode): (IContextMenuValue | null)[]
 
   /**
    * Allows the extension to add additional handling to the node before it is registered with **LGraph**