Python: Use BaseModel instead of KernelBaseModel in samples (#12224)

### Motivation and Context

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It's unnecessary to use `KernelBaseModel` in the samples.

### Description

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Switch to `BaseModel`.


### 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)
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raises no violations
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- [x] I didn't break anyone 😄

Author: TaoChenOSU

python/samples/getting_started_with_agents/multi_agent_orchestration/step4a_handoff_structured_inputs.py

@@ -3,12 +3,13 @@
 import asyncio
 from enum import Enum
 
+from pydantic import BaseModel
+
 from semantic_kernel.agents import Agent, ChatCompletionAgent, HandoffOrchestration, OrchestrationHandoffs
 from semantic_kernel.agents.runtime import InProcessRuntime
 from semantic_kernel.connectors.ai.open_ai import OpenAIChatCompletion
 from semantic_kernel.contents import AuthorRole, ChatMessageContent
 from semantic_kernel.functions import kernel_function
-from semantic_kernel.kernel_pydantic import KernelBaseModel
 
 """
 The following sample demonstrates how to create a handoff orchestration that can triage
@@ -37,7 +38,7 @@ class GitHubLabels(Enum):
     AGENT = "agent"
 
 
-class GithubIssue(KernelBaseModel):
+class GithubIssue(BaseModel):
     """Model representing a GitHub issue."""
 
     id: str
@@ -46,7 +47,7 @@ class GithubIssue(KernelBaseModel):
     labels: list[str] = []
 
 
-class Plan(KernelBaseModel):
+class Plan(BaseModel):
     """Model representing a plan for resolving a GitHub issue."""
 
     tasks: list[str]