fix: accept items with filename but no subfolder in isResultItem

Restores compatibility with custom nodes that only send filename without
subfolder. The ResultItemImpl constructor already falls back to ''.

Addresses review feedback:
https://github.com/Comfy-Org/ComfyUI_frontend/pull/9622#discussion_r2925170216

src/renderer/extensions/linearMode/flattenNodeOutput.test.ts

@@ -120,11 +120,26 @@ describe(flattenNodeOutput, () => {
     expect(result[0].mediaType).toBe('images')
   })
 
-  it('excludes partial ResultItem objects missing required fields', () => {
+  it('accepts items with filename but no subfolder', () => {
+    const output = {
+      images: [
+        { filename: 'valid.png', subfolder: '', type: 'output' },
+        { filename: 'no-subfolder.png' }
+      ]
+    } as unknown as NodeExecutionOutput
+
+    const result = flattenNodeOutput(['1', output])
+
+    expect(result).toHaveLength(2)
+    expect(result[0].filename).toBe('valid.png')
+    expect(result[1].filename).toBe('no-subfolder.png')
+    expect(result[1].subfolder).toBe('')
+  })
+
+  it('excludes items missing filename', () => {
     const output = {
       images: [
         { filename: 'valid.png', subfolder: '', type: 'output' },
-        { filename: 'no-subfolder.png' },
         { subfolder: '', type: 'output' }
       ]
     } as unknown as NodeExecutionOutput

src/stores/resultItemParsing.test.ts

@@ -119,11 +119,26 @@ describe(parseNodeOutput, () => {
     expect(result[0].mediaType).toBe('images')
   })
 
-  it('excludes partial ResultItem objects missing required fields', () => {
+  it('accepts items with filename but no subfolder', () => {
+    const output = {
+      images: [
+        { filename: 'valid.png', subfolder: '', type: 'output' },
+        { filename: 'no-subfolder.png' }
+      ]
+    } as unknown as NodeExecutionOutput
+
+    const result = parseNodeOutput('1', output)
+
+    expect(result).toHaveLength(2)
+    expect(result[0].filename).toBe('valid.png')
+    expect(result[1].filename).toBe('no-subfolder.png')
+    expect(result[1].subfolder).toBe('')
+  })
+
+  it('excludes items missing filename', () => {
     const output = {
       images: [
         { filename: 'valid.png', subfolder: '', type: 'output' },
-        { filename: 'no-subfolder.png' },
         { subfolder: '', type: 'output' }
       ]
     } as unknown as NodeExecutionOutput

src/stores/resultItemParsing.ts

@@ -5,12 +5,10 @@ import { ResultItemImpl } from '@/stores/queueStore'
 const METADATA_KEYS = new Set(['animated', 'text'])
 
 /**
- * Validates that an unknown value is a well-formed ResultItem with required
- * fields for constructing a previewable output.
+ * Validates that an unknown value is a well-formed ResultItem.
  *
- * Stricter than `zResultItem.safeParse()` because the Zod schema marks
- * `filename` and `subfolder` as optional (matching the wire format), but
- * a ResultItemImpl needs both to construct a valid URL.
+ * Requires `filename` (string) since ResultItemImpl needs it for a valid URL.
+ * `subfolder` is optional here — ResultItemImpl constructor falls back to ''.
  */
 function isResultItem(item: unknown): item is ResultItem {
   if (!item || typeof item !== 'object' || Array.isArray(item)) return false
@@ -18,7 +16,6 @@ function isResultItem(item: unknown): item is ResultItem {
   const candidate = item as Record<string, unknown>
 
   if (typeof candidate.filename !== 'string') return false
-  if (typeof candidate.subfolder !== 'string') return false
 
   if (
     candidate.type !== undefined &&