fix: force image parser json output (#15847)

### What problem does this PR solve?
Force image parser runtime output format to JSON so downstream chunking
reads OCR results from the JSON output and image parser chunks can be
displayed.

### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: Wang Qi <wangq8@outlook.com>

rag/flow/parser/parser.py

@@ -1055,7 +1055,7 @@ class Parser(ProcessBase):
 
         self.callback(random.randint(1, 5) / 100.0, "Start to work on an image.")
         conf = self._param.setups["image"]
-        self.set_output("output_format", conf["output_format"])
+        self.set_output("output_format", "json")
 
         img = Image.open(io.BytesIO(blob)).convert("RGB")