feat: raise descriptive error when proctoring provider cannot remove an attempt

[Anas12091101]

Description:

Makes proctored-exam attempt removal fail loudly and clearly when the proctoring provider errors, instead of hanging, returning an unhandled 500 (which rolled back the local delete), or silently doing a local-only reset.

• Timeout on the REST backend request so a slow/unavailable provider can't hang the request. Default 30s, overridable per backend via timeout in PROCTORING_BACKENDS.
• On failure, the pre_delete handler logs it and raises a typed BackendProviderCannotRemoveAttempt (HTTP 502) instead of a 500; the local delete is aborted so the attempt is preserved.
  • Unreachable / timeout → generic "temporarily unavailable, try again" message.
  • Provider HTTP error (4xx/5xx) → previously only logged while still deleting locally; now raises a provider-status-aware 502 and logs the raw provider response. On by default, disable per backend via raise_on_remove_error in PROCTORING_BACKENDS.
• Benign cases (e.g. attempt with no external_id) are unchanged — reset still proceeds.

Tests cover the timeout (default + configurable), the unreachable and provider-error paths (with/without body, and the opt-out), and preserving the provider message vs. the generic one.

Pre-Merge Checklist:

• Updated the version number in edx_proctoring/__init__.py and package.json if these changes are to be released.
• Described your changes in CHANGELOG.rst
• Confirmed Github reports all automated tests/checks are passing.
• Approved by at least one additional reviewer.

Post-Merge:

• Create a tag matching the new version number.

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