refactor: discriminate DownloadService union by supportsPauseResume

Split DownloadService into PausableDownloadService and
NonPausableDownloadService discriminated by supportsPauseResume.
Removes pause/resume stubs from cloud and browser providers; the
compiler now enforces narrowing before pause/resume calls.

Addresses review feedback:
https://github.com/Comfy-Org/ComfyUI_frontend/pull/11437#discussion_r3113480116

src/platform/downloads/providers/createBrowserDownloadService.ts

@@ -1,10 +1,10 @@
 import type {
   DownloadEntry,
-  DownloadService,
-  DownloadStartParams
+  DownloadStartParams,
+  NonPausableDownloadService
 } from '../types'
 
-export function createBrowserDownloadService(): DownloadService {
+export function createBrowserDownloadService(): NonPausableDownloadService {
   async function start(params: DownloadStartParams): Promise<DownloadEntry> {
     const anchorElement = document.createElement('a')
     anchorElement.href = params.url
@@ -23,8 +23,6 @@ export function createBrowserDownloadService(): DownloadService {
     }
   }
 
-  async function pause() {}
-  async function resume() {}
   async function cancel() {}
 
   function getAll(): DownloadEntry[] {
@@ -42,8 +40,6 @@ export function createBrowserDownloadService(): DownloadService {
   return {
     supportsPauseResume: false,
     start,
-    pause,
-    resume,
     cancel,
     getAll,
     getById,

src/platform/downloads/providers/createCloudDownloadService.ts

@@ -1,10 +1,10 @@
 import type {
   DownloadEntry,
-  DownloadService,
-  DownloadStartParams
+  DownloadStartParams,
+  NonPausableDownloadService
 } from '../types'
 
-export function createCloudDownloadService(): DownloadService {
+export function createCloudDownloadService(): NonPausableDownloadService {
   const entries = new Map<string, DownloadEntry>()
   const progressListeners = new Map<
     string,
@@ -33,8 +33,6 @@ export function createCloudDownloadService(): DownloadService {
     return entry
   }
 
-  async function pause() {}
-  async function resume() {}
   async function cancel() {}
 
   function getAll(): DownloadEntry[] {
@@ -61,8 +59,6 @@ export function createCloudDownloadService(): DownloadService {
   return {
     supportsPauseResume: false,
     start,
-    pause,
-    resume,
     cancel,
     getAll,
     getById,

src/platform/downloads/providers/createElectronDownloadService.ts

@@ -2,9 +2,9 @@ import type { DownloadProgressUpdate } from '@comfyorg/comfyui-electron-types'
 
 import type {
   DownloadEntry,
-  DownloadService,
   DownloadStartParams,
-  DownloadStatus
+  DownloadStatus,
+  PausableDownloadService
 } from '../types'
 
 import { electronAPI } from '@/utils/envUtil'
@@ -29,7 +29,7 @@ function toDownloadStatus(
   return isDownloadStatus(value) ? value : fallback
 }
 
-export async function createElectronDownloadService(): Promise<DownloadService> {
+export async function createElectronDownloadService(): Promise<PausableDownloadService> {
   const api = electronAPI()
   const downloadManager = api?.DownloadManager
   if (!downloadManager) {

src/platform/downloads/types.ts

@@ -34,12 +34,10 @@ export interface DownloadStartParams {
   tags?: string[]
 }
 
-export interface DownloadService {
+interface BaseDownloadService {
   /** Resolves once the download is accepted, not when it completes. */
   start(params: DownloadStartParams): Promise<DownloadEntry>
 
-  pause(id: string): Promise<void>
-  resume(id: string): Promise<void>
   cancel(id: string): Promise<void>
 
   getAll(): DownloadEntry[]
@@ -47,6 +45,23 @@ export interface DownloadService {
 
   /** Returns an unsubscribe function. */
   onProgress(id: string, cb: (entry: DownloadEntry) => void): () => void
-
-  readonly supportsPauseResume: boolean
 }
+
+export interface PausableDownloadService extends BaseDownloadService {
+  readonly supportsPauseResume: true
+  pause(id: string): Promise<void>
+  resume(id: string): Promise<void>
+}
+
+export interface NonPausableDownloadService extends BaseDownloadService {
+  readonly supportsPauseResume: false
+}
+
+/**
+ * Discriminated union by {@link supportsPauseResume}. Callers must narrow
+ * on the discriminant before calling {@link PausableDownloadService.pause}
+ * or {@link PausableDownloadService.resume}; the compiler enforces this.
+ */
+export type DownloadService =
+  | PausableDownloadService
+  | NonPausableDownloadService