fix: Implement missing run_moderation method in PromptGuardSafetyImpl (#3101)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
This PR addresses an issue where `PromptGuardSafetyImpl` was an
incomplete implementation of an abstract class. The class was missing
the required run_moderation method from its parent interface.


Currently, running `pre-commit` locally fails with the error below.

```
llama_stack/providers/inline/safety/prompt_guard/__init__.py:15: error: Cannot instantiate abstract class "PromptGuardSafetyImpl" with abstract attribute "run_moderation"  [abstract]
Found 1 error in 1 file (checked 410 source files)
```

This PR fixes the issue as follows

- Added the missing run_moderation method to PromptGuardSafetyImpl
- Method raises NotImplementedError with appropriate message indicating
this functionality is not implemented for PromptGuard
- This allows the class to be properly instantiated while clearly
indicating the limitation

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

Signed-off-by: Mustafa Elbehery <melbeher@redhat.com>

llama_stack/providers/inline/safety/prompt_guard/prompt_guard.py

@@ -64,6 +64,9 @@ class PromptGuardSafetyImpl(Safety, ShieldsProtocolPrivate):
 
         return await self.shield.run(messages)
 
+    async def run_moderation(self, input: str | list[str], model: str):
+        raise NotImplementedError("run_moderation not implemented for PromptGuard")
+
 
 class PromptGuardShield:
     def __init__(