Python: Fix type error in kernel server.py (#3185)

### Description

This PR fixes #3123.
The problem was in line 147 of kernel_server.py :
[Line147](https://github.com/microsoft/semantic-kernel/blob/bdc517d0f80e0f0896a8f1fc01d4327848c74937/samples/python/kernel_http_server/utils/kernel_server.py#L147C9-L147C14)

The plan object here is plan.Plan, which is incorrect. I believe it
should be basic_planner.Plan, here :
[plan](https://github.com/microsoft/semantic-kernel/blob/bdc517d0f80e0f0896a8f1fc01d4327848c74937/python/semantic_kernel/planning/basic_planner.py#L12C7-L12C11)

### Contribution Checklist

- [ ] 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
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

---------

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>

samples/python/kernel_http_server/utils/kernel_server.py

@@ -6,7 +6,7 @@
 from semantic_kernel import ContextVariables
 from semantic_kernel.memory import VolatileMemoryStore
 from semantic_kernel.planning.basic_planner import BasicPlanner
-from semantic_kernel.planning.plan import Plan
+from semantic_kernel.planning.basic_planner import Plan
 from utils.kernel_utils import create_kernel_for_request
 from utils.ask import Ask, AskResult, AskInput