Handle network errors in useSubmissionsList test run fetch

[iHiD]

Closes #8465
Closes #8467

Summary

• Add .catch() handler to the test run fetch in useSubmissionsList that removes the submission from the list on network failure, resetting the UI to a clean state instead of leaving it stuck in a loading/pending state
• The Sentry beforeSend filter already drops "Failed to fetch" errors (commit 741e5e7), so no new Sentry filtering is needed
• TypeError: Failed to fetch (exercism.org) #8465 (request-query.ts) needs no code change — React Query already handles fetch errors via its built-in retry/error state

cc @dem4ron

Test plan

• yarn test passes (160/160 suites, 1550/1550 tests)
• Verify in browser: when a network error occurs during test run fetch, the editor resets to a clean state rather than showing a stuck loading indicator

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