Python: Debug logging for StepwisePlanner's next step thought (#4333)

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The next_step.thought of StepwisePlanner is not being output in the
debug logs.

### Description

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### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
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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 😄

Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

python/semantic_kernel/planning/stepwise_planner/stepwise_planner.py

@@ -157,7 +157,7 @@ async def execute_plan_async(self, context: SKContext) -> SKContext:
 
                     return context
 
-                logger.debug("Thoughts: {next_step.thought}")
+                logger.debug(f"Thoughts: {next_step.thought}")
 
                 if not is_null_or_empty(next_step.action):
                     logger.info(f"Action: {next_step.action}. Iteration: {i+1}.")