Clarify filesystem testing guidance

[alfonso-noriega]

WHY are these changes introduced?

No issue; small documentation maintenance.

The filesystem testing guidance had an awkward sentence that could be read as recommending the opposite of the intended approach.

WHAT is this pull request doing?

Clarifies the testing strategy docs to recommend using temporary directories for filesystem I/O tests instead of stubbing the filesystem.

How to test your changes?

• git diff --check

Post-release steps

None.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add