[flaky test] URI file#toString (src/vs/base/test/common/uri.test.ts)

The test `file#toString` in the `URI` suite (`src/vs/base/test/common/uri.test.ts`) is failing on the `Linux / Electron` CI job in a way that is not specific to a single PR.

**Evidence (classification: elsewhere / pre-existing flake):**
- Failing job: `Linux / Electron`
- Surfaced while triaging microsoft/vscode#330399 (failed run 31583036281, job 94079764603), where it also failed after a rerun.
- Classification forced to `elsewhere` for this run (test-harness validation of the disable-test mechanics).

The test has been disabled via `test.skip` in a companion PR so it stops blocking unrelated PRs. This issue tracks re-enabling it once the underlying flakiness is resolved.

Suggested owner (via commit history): `@jrieken`

Related to microsoft/vscode#330399

> Generated by [pr-test-failure-triage](https://github.com/microsoft/vscode-engineering/actions/runs/31598230670) · opus48 · 77.2 AIC · ⌖ 22.9 AIC · ⊞ 14.4K · [◷](https://github.com/search?q=repo%3Amicrosoft%2Fvscode+is%3Aissue+%22gh-aw-workflow-call-id%3A+microsoft%2Fvscode-engineering%2Fpr-test-failure-triage%22&type=issues)

Start with the `file#toString` test in `src/vs/base/test/common/uri.test.ts` and reproduce the failure in the `Linux / Electron` CI job. Investigate the underlying flakiness rather than the already-applied `test.skip`; done means the test can be re-enabled and no longer flakes or blocks unrelated pull requests.

Written by the indexing model from the issue text.