Expose helper for resolving python-env search paths (getAllExtraSearchPaths)

@eleanorjboyd がすでに取り組んでいます。

2025年11月11日 から。

### Summary
Extensions that integrate with ms-python.vscode-python-envs need the canonical list of user-configured search paths. Today that logic lives in src/managers/common/nativePythonFinder.ts#getAllExtraSearchPaths, but it isn’t exposed through the public API, so consumers have to re-implement a fairly large helper.

### What we’d like
Export a function (or add an API surface) that returns the same array of search directories the native finder uses—covering legacy python.venvPath / venvFolders, python-env.globalSearchPaths, workspaceSearchPaths, validation, and relative-path resolution across multi-root workspaces.

### Why
In order to use this for another environment plugin, such as the pixi env manager (https://github.com/renan-r-santos/pixi-code), one would have to copy over that code verbatim so it can stay behaviourally aligned with ms-python.vscode-python-envs. Keeping two copies in sync is brittle; a shared helper would make it much easier to keep the behaviour consistent by default.

Happy to send a PR once there’s agreement on where such an export should live.