Add prop descriptions for CollapsiblePanel hidden_until_found and keep_mounted

Summary

Added missing prop descriptions for hidden_until_found and keep_mounted properties in the CollapsiblePanel class. The descriptions were sourced directly from the official Base UI GitHub repository to ensure accuracy and consistency with the upstream component documentation.

Changes:

• Added description for hidden_until_found: Explains browser's built-in page search functionality and hidden="until-found" behavior
• Added description for keep_mounted: Explains DOM mounting behavior when panel is hidden
• Updated both .py and .pyi files for consistency
• Descriptions follow existing comment patterns in the codebase

Review & Testing Checklist for Human

• Verify prop descriptions match Base UI documentation - Cross-reference with https://github.com/mui/base-ui/blob/master/packages/react/src/collapsible/panel/CollapsiblePanel.tsx to ensure accuracy
• Check consistency between .py and .pyi files - Ensure both files have identical descriptions for the same props
• Test component functionality - Run demo app to verify collapsible component still works properly (though comment changes shouldn't affect functionality)

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    A["reflex_ui/components/base/collapsible.py"]:::major-edit
    B["reflex_ui/components/base/collapsible.pyi"]:::major-edit
    C["CollapsiblePanel class"]:::context
    D["hidden_until_found prop"]:::context
    E["keep_mounted prop"]:::context
    F["Base UI GitHub repo"]:::context

    A --> C
    B --> C
    C --> D
    C --> E
    F --> |"sourced descriptions"| D
    F --> |"sourced descriptions"| E

    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end

    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

Notes

• This completes the prop documentation that was missing from the original collapsible component implementation
• The demo and transition classes mentioned in the original task were already implemented in previous work
• All pre-commit hooks passed (ruff check, ruff format, codespell, pyright)
• Session requested by Carlos Cutillas (carlos@reflex.dev)
• Link to Devin run: https://app.devin.ai/sessions/ddfe92d2015544f1b8bef1ec7c5f45da