test: update extraction service prompt expectations

Updates the test expectations in `src/service/extraction.test.ts` to match the actual prompt content used in `src/service/extraction.ts`. The previous test was asserting the presence of specific instructions regarding cropping artifacts that are no longer present in the codebase. The updated test now verifies the presence of the task name and the instruction to return only the image.

src/service/extraction.test.ts

@@ -109,7 +109,7 @@ describe('ExtractionService', () => {
     const callArgs = generateImageMock.mock.calls[0] as unknown as any[]
     const prompt = callArgs[0] as string
 
-    expect(prompt).toContain('CRITICAL: Detect and remove any straight lines, black borders, or frame-like artifacts')
-    expect(prompt).toContain('These are cropping artifacts and MUST be removed')
+    expect(prompt).toContain('Task: Asset Refinement')
+    expect(prompt).toContain('Return only the image.')
   })
 })