Refactor storage.ts into testable modules and add unit tests

[EhabY]

Closes #588

This PR refactors storage.ts into small, isolated modules that are straightforward to unit test (with mocks). It also upgrades vitest to a version that plays nicely with VS Code extensions, so we can view coverage, run and debug tests directly in VS Code.

VS Code Test Explorer:

Screencast.from.2025-09-22.16-54-03.webm

Why mock vscode?

With the current VS Code Extension API, we essentially have two testing layers:

• Unit tests (mocked vscode) : Validate our module's behavior independently of the VS Code API and external calls (e.g., Axios). These are fast, reliable, and cover most logic.
• Integration/E2E tests (real VS Code): Exercise "real" user flows by launching VS Code and, eventually, a server. These are valuable but more limited, since UI interaction is hard to automate and not all code paths are easily reachable.

By mocking vscode for unit tests, we keep feedback tight and deterministic, while reserving integration tests for end-to-end scenarios that require the actual runtime.

[code-asher]

Excellent! The only thing I think maybe needs a double-check before merging is the useCustom with the proposed API. If you want to merge and address the rest later, I think that is chill, or you could do it here, up to you!