fix(copilot): match the oversized-image placeholder in the read-size gate

isOversizedReadPlaceholder tested for '[Image too large:', a prefix no code
emitted — the image path's only placeholder was '[Image unavailable:'. The new
source-cap placeholder is the first real one, so point the gate at it and fix
the synthetic string in the test to match what the reader actually returns.

Author: waleedlatif1

apps/sim/lib/copilot/tools/handlers/vfs.test.ts

@@ -182,7 +182,7 @@ describe('vfs handlers oversize policy', () => {
   it('fails oversized image placeholder when image exceeds size limit', async () => {
     const vfs = makeVfs()
     vfs.readFileContent.mockResolvedValue({
-      content: '[Image too large: huge.png (10.0MB, limit 5MB)]',
+      content: '[Image too large to read inline: huge.png (26214401 bytes, limit 26214400)]',
       totalLines: 1,
     })
     getOrMaterializeVFS.mockResolvedValue(vfs)

apps/sim/lib/copilot/tools/handlers/vfs.ts

@@ -79,7 +79,7 @@ function serializedResultSize(value: unknown): number {
 function isOversizedReadPlaceholder(content: string): boolean {
   return (
     content.startsWith('[File too large to display inline:') ||
-    content.startsWith('[Image too large:') ||
+    content.startsWith('[Image too large to read inline:') ||
     content.startsWith('[Compiled artifact too large:')
   )
 }