Python: Throw exception on image error (#12410)

### Motivation and Context

### Description
This small PR throws the forgotten exception on image error.

### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
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raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Signed-off-by: Emmanuel Ferdman <[email protected]>

Author: emmanuel-ferdman

python/semantic_kernel/agents/open_ai/responses_agent_thread_actions.py

@@ -699,7 +699,9 @@ def _prepare_chat_history_for_request(
                             image_url = str(content.uri)
 
                         if not image_url:
-                            ValueError("ImageContent must have either a data_uri or uri set to be used in the request.")
+                            raise ValueError(
+                                "ImageContent must have either a data_uri or uri set to be used in the request."
+                            )
 
                         contents.append({"type": "input_image", "image_url": image_url})
                         response_inputs.append({"role": original_role, "content": contents})