PR: Fix extension install path not changeable (#3193)

[tay0thman]

Added functions to ensure custom extension paths are registered and to get the default extension directory. Updated the custom section for extension installation to handle new paths and states.

PR: Fix extension install path not changeable (#3193)

Fixes #3193 — The Extension Manager's "Pick installation path" button was grayed out when selecting a catalog extension to install, and the auto-selected default path was non-deterministic. Additionally, picking a custom folder that wasn't already in the user's Custom Extension Directories would silently fail after reload because the folder was never registered as a search path.ct.

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Checklist

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• [ ok] Code follows the PEP 8 style guide.
• [ ok] Code has been formatted with Black using the command:

  pipenv run black {source_file_or_directory}

• [ ok] Changes are tested and verified to work as expected.

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Related Issues

Root Cause

The Feb 15 UI refactoring (commit ec2eb76) replaced the old context-menu dropdown install flow with a text field + "Pick" button, but introduced three bugs: _update_add_custom_section_for_selection() unconditionally set path_custom_ext_b.IsEnabled = False, locking out path selection for uninstalled extensions
get_thirdparty_ext_root_dirs() collected paths in a Python set(), making [0] non-deterministic — users got their server path instead of the local default
install_custom_extension() never registered the chosen install directory in pyRevit's config, so extensions installed to new folders weren't discovered after reload

Additional Notes

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