[Python] Update customized code after generating code

### 🧑‍💻 Who is experiencing this?
service team/SDK language owners

### 🎯 What's their Job-to-be-done
What was the user trying to accomplish when they hit the problem?
update SDK with customized code

### 💥 What is the problem?
Describe the problem as clearly and specifically as possible.
What went wrong from the user’s perspective?
It requires much manual efforts to update customized code. For example, if python SDK devs want to customize generated code, they need to add code in _patch.py like https://github.com/Azure/azure-sdk-for-python/blob/b59df36163bc9b3278e1043055b61c8d44d62f90/sdk/face/azure-ai-vision-face/azure/ai/vision/face/aio/_patch.py#L267-L302. However, if service teams introduces typespce change which result in API change, they need to update the customized code to adopt the change.

### 🧭 Where does the problem occur?
List the environment(s) and tool(s) in which the user was operating when they encountered the problem.

- SDK language: Python, data-plane
- Tooling: Intellij/VS Code
- Runtime environment: Local

Start with the referenced Python SDK customization in azure/ai/vision/face/aio/_patch.py, especially the linked customized-code section, then trace how a typespec change affects generated and customized code. Done should reduce the manual effort required for Python data-plane SDK owners to update customized code after an API change.

Written by the indexing model from the issue text.