Fix: Use AUTOGENSTUDIO_UI_PATH for custom UI location

[abhishekmishra2903]

The AutoGen Studio server was previously hardcoding the UI path to a subdirectory within its own package (autogenstudio/web/ui). This prevented you from serving UI assets from a custom compiled location, even when setting the AUTOGENSTUDIO_UI_PATH environment variable.

This commit modifies the AppInitializer in autogenstudio.web.initialization to:

1. Check for the AUTOGENSTUDIO_UI_PATH environment variable.
2. If set and the specified directory exists, use this path as the ui_root.
3. Log which UI path (default or custom) is being used.
4. Adjust directory creation logic to avoid errors if a custom UI path is provided.

This change allows you to correctly point AutoGen Studio to your custom UI builds, resolving the issue where the server would return 404 errors for the UI despite AUTOGENSTUDIO_UI_PATH being set.

Why are these changes needed?

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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