Fix expander accessor return type annotation

[Nikhil-jaiswal007]

Describe your changes

Fixed the return type annotation for the expander accessor in lib/streamlit/testing/v1/app_test.py to correctly reflect the expected type signature.

Screenshot or video (only for visual changes)

N/A - This is a purely backend typing update.

GitHub Issue Link (if applicable)

Fixes #[14528] (Note: If you were solving a specific issue on GitHub for this type hint, put the number here so it auto-links. If you just found this typo yourself and there is no issue, you can just delete this whole line).

Testing Plan

Explanation of why no additional tests are needed: This PR only updates a type annotation within the testing framework itself (app_test.py), so no new runtime features were added that require new tests. Local pytest and type checking pass.

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